Is my cosmology missing anything?

[Scarey Nerd]

I've been running through my cosmology for my campaign setting, and I'm approaching a final draft of it, but I need some advice on whether I'm missing anything. The 5e DMG says that at minimum you should have a plane for fiends, one for celestials, one for elementals, one for deities, one for an afterlife, a transitive plane, and ones for spells and/or monsters that use the astral and/or ethereal plane to function. With the exception of Elementals, who in my world are simply beings created from the elements either by a mage's spell or by interaction with Wild Magic, I think I have all of them.

Spoiler: Planes of Saere

Show

The Mortal Realm: This is the material plane, where the main game world (Saere) exists. Sort of self-explanatory.

The Ætherium: A formless sea of thought and dream, the Ætherium is the nothing from which the rest of creation was constructed; it is where Gods are born and die, and where the Planes spring into existence. The Ætherium is a transitive plane that naturally touches all other planes, and is the equivalent of the Astral Plane.

The Realm of Shadow: This is another transitive plane, and functions in a similar way to both the Shadowfell and the Ethereal Plane. When a mortal dies, its soul is brought to the Realm of Shadow, where it journeys down the great river that exists there, which in turn goes by many names. The soul bobs along the river until it reaches its end, whereby it is deposited in the afterlife it is supposed to go to, be it the realm of a God they worshipped or what have you. Undead are made by binding spirits from the Realm of Shadow, the stubborn ones that refuse to truly pass on and instead claw their way back up the river. Additionally, at the border between the Realm of Shadow and the Mortal Realm, it is possible to see the latter from the former, much like the Ethereal Plane, and additionally if one knows how one can travel along the border to move through the material world faster than if you travelled the distance physically. Spells that use the Ethereal Plane use the Realm of Shadow instead.

The Twilight Lands: Not a single plane, but rather a collection of planes that border each other in a chaotic and eclectic arrangement, like a pile of soap bubbles where the bubbles are constantly shifting places with each other. Each demiplane is inhabited by a Fey court, a Fey race or similar. In the Mortal Realm, there are fixed places that function as portals to the Twilight Lands, but the actual demiplane you travel to when the portal is active (e.g. a full moon, etc) changes regularly, sometimes taking you to and from the same demiplane for a month, sometimes changing every few minutes. If you've ever read the Faraway Tree by Enid Blyton, imagine that setup, only less ordered.

Heaven/Hell: Both one plane and two at the same time, this realm is home to Angels and Demons; Celestials and Fiends. After a huge event that shook the very foundations of the world many thousands of years ago, the mortal races were tearing each other apart in a dark time for all concerned. Some of the greatest mages of the age came together to attempt to quell the darkness in the hearts of the sentient peoples of Saere, and weaved a great spell to do so, but it went rather spectacularly wrong, and instead split each persons soul into 3 equal parts: the most Good part, which became an Angel, the most Evil, which became a Demon, and the rest which stayed with the mortal in question, so rather than swinging from pure goodness to pure evil all the time, people could be a little more neutral. The Angels and Demons fought a bitter war, naturally, that threatened to destroy everything, so the mages reconvened some time later and banished the whole lot of them to another plane, where they still fight their war from now until the end of time.

The Crossroads: A chaotic plane with portals, doorways and staircases leading to every other realm. If you've seen Jonathon Strange & Mr Norell, imagine the Kings Roads. You never know what door you might open one day to find a staircase descending upwards.

The Realms of the Gods: Not really connected, this is just a group term for the many planes of existence that exist to house the Gods. Some are bigger than others, some are smaller, depending on the worship that God receives. Most afterlives come under this term.

What do you guys think? Have I missed out any crucial planes? Was my decision to remove the Elemental Planes flawed?

[Xuc Xac]

The 5e DMG says that at minimum you should have a plane for fiends, one for celestials, one for elementals, one for deities, one for an afterlife, a transitive plane, and ones for spells and/or monsters that use the astral and/or ethereal plane to function.

Yes, that's really good advice, but they forgot to mention shoes for fish. At a minimum, your fish will need one pair of running shoes for playing sports, one pair of casual shoes, and one pair of dress shoes for job interviews and special occasions. You may also want a special pair of trainers for when your fish is riding its bicycle.

[LudicSavant]

The 5e DMG says that at minimum you should have a plane for fiends, one for celestials, one for elementals, one for deities, one for an afterlife, a transitive plane, and ones for spells and/or monsters that use the astral and/or ethereal plane to function.

The 5e DMG can stuff it. There are examples of major published D&D settings that don't have that (Eberron, for example, doesn't divvy up planes this way. Heck, some fiends are from the material plane). You should not take its advice seriously.

Yes, that's really good advice, but they forgot to mention shoes for fish. At a minimum, your fish will need one pair of running shoes for playing sports, one pair of casual shoes, and one pair of dress shoes for job interviews and special occasions. You may also want a special pair of trainers for when your fish is riding its bicycle.

Xuc Xac is giving the 5e DMG advice the amount of seriousness it deserves.

[Max_Killjoy]

I've been running through my cosmology for my campaign setting, and I'm approaching a final draft of it, but I need some advice on whether I'm missing anything. The 5e DMG says that at minimum you should have a plane for fiends, one for celestials, one for elementals, one for deities, one for an afterlife, a transitive plane, and ones for spells and/or monsters that use the astral and/or ethereal plane to function. With the exception of Elementals, who in my world are simply beings created from the elements either by a mage's spell or by interaction with Wild Magic, I think I have all of them.

Spoiler: Planes of Saere

Show

The Mortal Realm: This is the material plane, where the main game world (Saere) exists. Sort of self-explanatory.

The Ætherium: A formless sea of thought and dream, the Ætherium is the nothing from which the rest of creation was constructed; it is where Gods are born and die, and where the Planes spring into existence. The Ætherium is a transitive plane that naturally touches all other planes, and is the equivalent of the Astral Plane.

The Realm of Shadow: This is another transitive plane, and functions in a similar way to both the Shadowfell and the Ethereal Plane. When a mortal dies, its soul is brought to the Realm of Shadow, where it journeys down the great river that exists there, which in turn goes by many names. The soul bobs along the river until it reaches its end, whereby it is deposited in the afterlife it is supposed to go to, be it the realm of a God they worshipped or what have you. Undead are made by binding spirits from the Realm of Shadow, the stubborn ones that refuse to truly pass on and instead claw their way back up the river. Additionally, at the border between the Realm of Shadow and the Mortal Realm, it is possible to see the latter from the former, much like the Ethereal Plane, and additionally if one knows how one can travel along the border to move through the material world faster than if you travelled the distance physically. Spells that use the Ethereal Plane use the Realm of Shadow instead.

The Twilight Lands: Not a single plane, but rather a collection of planes that border each other in a chaotic and eclectic arrangement, like a pile of soap bubbles where the bubbles are constantly shifting places with each other. Each demiplane is inhabited by a Fey court, a Fey race or similar. In the Mortal Realm, there are fixed places that function as portals to the Twilight Lands, but the actual demiplane you travel to when the portal is active (e.g. a full moon, etc) changes regularly, sometimes taking you to and from the same demiplane for a month, sometimes changing every few minutes. If you've ever read the Faraway Tree by Enid Blyton, imagine that setup, only less ordered.

Heaven/Hell: Both one plane and two at the same time, this realm is home to Angels and Demons; Celestials and Fiends. After a huge event that shook the very foundations of the world many thousands of years ago, the mortal races were tearing each other apart in a dark time for all concerned. Some of the greatest mages of the age came together to attempt to quell the darkness in the hearts of the sentient peoples of Saere, and weaved a great spell to do so, but it went rather spectacularly wrong, and instead split each persons soul into 3 equal parts: the most Good part, which became an Angel, the most Evil, which became a Demon, and the rest which stayed with the mortal in question, so rather than swinging from pure goodness to pure evil all the time, people could be a little more neutral. The Angels and Demons fought a bitter war, naturally, that threatened to destroy everything, so the mages reconvened some time later and banished the whole lot of them to another plane, where they still fight their war from now until the end of time.

The Crossroads: A chaotic plane with portals, doorways and staircases leading to every other realm. If you've seen Jonathon Strange & Mr Norell, imagine the Kings Roads. You never know what door you might open one day to find a staircase descending upwards.

The Realms of the Gods: Not really connected, this is just a group term for the many planes of existence that exist to house the Gods. Some are bigger than others, some are smaller, depending on the worship that God receives. Most afterlives come under this term.

What do you guys think? Have I missed out any crucial planes? Was my decision to remove the Elemental Planes flawed?

Instead of looking at 5e's "cosmology by numbers" and trying to fill in the blanks, you should start from the cultures that exist in your setting, decide what faiths, beliefs, myths, and legends mesh well with those cultures, and then work "back" to your cosmology such that it meshes with all that.

If you have "competing" religions, decide how those "conflicting truths" reconcile, and if your cosmology smooths that process.

I'll look at your posted cosmology this weekend when I get some free time and give some feedback.

[LudicSavant]

I've been running through my cosmology for my campaign setting, and I'm approaching a final draft of it, but I need some advice on whether I'm missing anything. The 5e DMG says that at minimum you should have a plane for fiends, one for celestials, one for elementals, one for deities, one for an afterlife, a transitive plane, and ones for spells and/or monsters that use the astral and/or ethereal plane to function. With the exception of Elementals, who in my world are simply beings created from the elements either by a mage's spell or by interaction with Wild Magic, I think I have all of them.

Spoiler: Planes of Saere

Show

The Mortal Realm: This is the material plane, where the main game world (Saere) exists. Sort of self-explanatory.

The Ætherium: A formless sea of thought and dream, the Ætherium is the nothing from which the rest of creation was constructed; it is where Gods are born and die, and where the Planes spring into existence. The Ætherium is a transitive plane that naturally touches all other planes, and is the equivalent of the Astral Plane.

The Realm of Shadow: This is another transitive plane, and functions in a similar way to both the Shadowfell and the Ethereal Plane. When a mortal dies, its soul is brought to the Realm of Shadow, where it journeys down the great river that exists there, which in turn goes by many names. The soul bobs along the river until it reaches its end, whereby it is deposited in the afterlife it is supposed to go to, be it the realm of a God they worshipped or what have you. Undead are made by binding spirits from the Realm of Shadow, the stubborn ones that refuse to truly pass on and instead claw their way back up the river. Additionally, at the border between the Realm of Shadow and the Mortal Realm, it is possible to see the latter from the former, much like the Ethereal Plane, and additionally if one knows how one can travel along the border to move through the material world faster than if you travelled the distance physically. Spells that use the Ethereal Plane use the Realm of Shadow instead.

The Twilight Lands: Not a single plane, but rather a collection of planes that border each other in a chaotic and eclectic arrangement, like a pile of soap bubbles where the bubbles are constantly shifting places with each other. Each demiplane is inhabited by a Fey court, a Fey race or similar. In the Mortal Realm, there are fixed places that function as portals to the Twilight Lands, but the actual demiplane you travel to when the portal is active (e.g. a full moon, etc) changes regularly, sometimes taking you to and from the same demiplane for a month, sometimes changing every few minutes. If you've ever read the Faraway Tree by Enid Blyton, imagine that setup, only less ordered.

Heaven/Hell: Both one plane and two at the same time, this realm is home to Angels and Demons; Celestials and Fiends. After a huge event that shook the very foundations of the world many thousands of years ago, the mortal races were tearing each other apart in a dark time for all concerned. Some of the greatest mages of the age came together to attempt to quell the darkness in the hearts of the sentient peoples of Saere, and weaved a great spell to do so, but it went rather spectacularly wrong, and instead split each persons soul into 3 equal parts: the most Good part, which became an Angel, the most Evil, which became a Demon, and the rest which stayed with the mortal in question, so rather than swinging from pure goodness to pure evil all the time, people could be a little more neutral. The Angels and Demons fought a bitter war, naturally, that threatened to destroy everything, so the mages reconvened some time later and banished the whole lot of them to another plane, where they still fight their war from now until the end of time.

The Crossroads: A chaotic plane with portals, doorways and staircases leading to every other realm. If you've seen Jonathon Strange & Mr Norell, imagine the Kings Roads. You never know what door you might open one day to find a staircase descending upwards.

The Realms of the Gods: Not really connected, this is just a group term for the many planes of existence that exist to house the Gods. Some are bigger than others, some are smaller, depending on the worship that God receives. Most afterlives come under this term.

What do you guys think? Have I missed out any crucial planes? Was my decision to remove the Elemental Planes flawed?

Removing the elemental planes is fine. You shouldn't be too worried about whether or not your cosmology meets a paint-by-numbers checklist. As long as you can answer questions like "how do the planar spells work?" you're good. You should instead be worrying about creating a compelling world with fertile ground for stories and characters to spring from and exciting places for adventures to unfold in.

To that end, the implication of celestials/fiends being partner to some material plane individual has potential, as do the conditional paths to faerie realms and the notion of walking the border between life and death for ethereal effects; it would be cool to see these ideas refined or expanded upon. Your transitive planes, deities planes, and river of souls, on the other hand, seem like they're just there to check boxes; at least as presented here.

[Scarey Nerd]

I totally agree with the recurring point you guys have made that I should be focusing on how the planes fit into the world rather than just ticking boxes - that is, in fact, what I've done, and each plane had a specific place in the cosmology even when I was creating the world with 3.5e in mind and didn't have much in the way of a list of suggestions. The only exception to that is the Ætherium, which I added in because I figured I needed an astral plane equivalent due to things like the Warlock Seeker patron, certain spells, etc.; that being said, it makes total sense in-universe to me because of my Small Gods-approach to the birth of deities, and my thoughts about what planes actually are metaphysically speaking.

I am interested by what Max_Killjoy means by how a cosmology could "smooth" the conflicting realities different religions present, which I confess I hadn't considered much before. I came up with a standard pantheon for each race, but only in this last couple of months have I decided "actually, all humans worshipping the same pantheon does NOT sit right with me", though I'm keeping the racial pantheon style for the other races for now at least. So to that end I probably will need to start considering how the cosmology is affected by other religions, as belief = creation to a point. So that's something I'll definitely keep in mind.

With regards to shoes for fish, in my world for simplicity's sake (and because I rarely run campaigns that involve going over bodies of water), all fish in my world wear a particular style of boot, rather than going with the standard multi-shoe approach. [/s]

[Max_Killjoy]

I am interested by what Max_Killjoy means by how a cosmology could "smooth" the conflicting realities different religions present, which I confess I hadn't considered much before. I came up with a standard pantheon for each race, but only in this last couple of months have I decided "actually, all humans worshipping the same pantheon does NOT sit right with me", though I'm keeping the racial pantheon style for the other races for now at least. So to that end I probably will need to start considering how the cosmology is affected by other religions, as belief = creation to a point. So that's something I'll definitely keep in mind.

What I meant was, if different religions (forget the pantheons for a moment) tell conflicting stories about the origin of the world, or the inherent underlying structure of the world ("... and then it's just elephants standing on elephants all the way down"), does the actual nature of reality allow for this somehow? Or are these religions not making a factual (at least partially, at the core) claim at all? Are they all telling variations on the same story that only seem contradictory (think Rashomon, or blind-men-and-elephant)? Are the religions telling the story has passed down from their deities, and if so, is it actually that none of those deities created the world and they're all "telling stories"? Or are the deities the product of human belief in a magic-exists setting, thus leaving the deities to remember with conviction all these things that the creation myths and legendary stories say of them?

[Name1]

When it comes to creation myths, Lords of Madness implies that the deities simply don't know it on grounds of them not having been there. This is actually a known fact by some scholars. How people still believe in creation myths despite that? Denial.

Seriously, the question of conflicting creation-stories can simply be solved by saying that the people believing in a particular one are simply to stubborn to admit that it's wrong, evidence be damned.

[ace rooster]

While I would treat the DMG fairly lightly, there are definitely mechanics that the cosmology interacts with that do need to be cleared up. In essence, you have check boxes for functions, rather than locations. Calling and summoning type effects are a big one. Having angels and demons come about after mortals is dangerous, because it makes if very hard to limit large scale weaponisation of outsiders by mortals. I don't know if 5th ed has calling, but I imagine that planar travel is a thing, and being able to go and get them works the same. Your setting needs something maintaining the status quo, and a cabal of wizards is not a good solution IMHO. It has the same problem as powerful active gods, in that any real problem is not the PC's problem. I suppose the banishment could be ongoing, and independent of the mages, which does open up huge adventure hooks if it is failing. Failing infrastructure of the universe is a classic adventure.

What happens on death is the big function that needs to be cleared up. You have that the soul leaves the body and travels down the river. It then presumably ends up at one of your afterlives, but you don't mention what format that occurs in. In particular, what happens if a live person plane shifts over and tries to hit them with a stick? What about asking them questions? Does somebody who is ethereal nearby when somebody dies have access to their soul, in such a way as to be able to trap, question, or even destroy it? What happens on the 'death' of an angel?

The cosmology is not just fluff setting building, which is why it needs to be thought about even for games purely on the material. I'm not getting at you particularly in this, as this stuff is not done well in the standard setting. Understanding the cosmology should be enough for a DM to be able to adjudicate consistently, (and predictibly for the players) when a player says something like "we search for the soul of the farmer to ask them what happened".

[Scarey Nerd]

What I meant was, if different religions (forget the pantheons for a moment) tell conflicting stories about the origin of the world, or the inherent underlying structure of the world ("... and then it's just elephants standing on elephants all the way down"), does the actual nature of reality allow for this somehow? Or are these religions not making a factual (at least partially, at the core) claim at all? Are they all telling variations on the same story that only seem contradictory (think Rashomon, or blind-men-and-elephant)? Are the religions telling the story has passed down from their deities, and if so, is it actually that none of those deities created the world and they're all "telling stories"? Or are the deities the product of human belief in a magic-exists setting, thus leaving the deities to remember with conviction all these things that the creation myths and legendary stories say of them?

The latter theory is absolutely what I'm going for. Intense belief interacts with the Ætherium, shaping reality, which is how Gods are born. A culture believes in a God, so the God exists, and the legends are true. However, if a culture believes that their God created the world and when the God dies the world will be destroyed, that does not mean that it will actually happen - instead, with lack of belief the God will cease to exist, and the world will keep spinning because the world does exist and therefore must continue to exist, as without mortal belief it can't keep existing. It sounds weird and a paradoxical interaction of physical and metaphysical, but to me at least it seems true to the nature of belief, i.e. that believing in something hard enough can make it appear to be true, even if the correlation and causation weren't related originally.

While I would treat the DMG fairly lightly, there are definitely mechanics that the cosmology interacts with that do need to be cleared up. In essence, you have check boxes for functions, rather than locations. Calling and summoning type effects are a big one. Having angels and demons come about after mortals is dangerous, because it makes if very hard to limit large scale weaponisation of outsiders by mortals. I don't know if 5th ed has calling, but I imagine that planar travel is a thing, and being able to go and get them works the same. Your setting needs something maintaining the status quo, and a cabal of wizards is not a good solution IMHO. It has the same problem as powerful active gods, in that any real problem is not the PC's problem. I suppose the banishment could be ongoing, and independent of the mages, which does open up huge adventure hooks if it is failing. Failing infrastructure of the universe is a classic adventure.

What happens on death is the big function that needs to be cleared up. You have that the soul leaves the body and travels down the river. It then presumably ends up at one of your afterlives, but you don't mention what format that occurs in. In particular, what happens if a live person plane shifts over and tries to hit them with a stick? What about asking them questions? Does somebody who is ethereal nearby when somebody dies have access to their soul, in such a way as to be able to trap, question, or even destroy it? What happens on the 'death' of an angel?

The cosmology is not just fluff setting building, which is why it needs to be thought about even for games purely on the material. I'm not getting at you particularly in this, as this stuff is not done well in the standard setting. Understanding the cosmology should be enough for a DM to be able to adjudicate consistently, (and predictibly for the players) when a player says something like "we search for the soul of the farmer to ask them what happened".

Don't worry, I don't think you're getting at me; in fact the best way I improve my ideas (or have some in the first place in a lot of cases) is by other people asking me questions. With regards to Angels and Demons, I'm not sure what you mean by weaponisation of Outsiders by mortals - I'm going to assume you mean mortals summoning outsiders for their own purposes, like a cult summoning a demon to unleash upon their enemies. While the DMG has spells for Summoning, Calling, Planar Travel etc, and the players can absolutely use those spells as they need, in practice the wider world is for the most part not privy to that magic. For Wizards, that means that those with the spell in their spellbook are rare in the extreme; for Sorcerers, that means that those spells only rarely awaken in Sorcerers; for Warlocks, that means it is usually not in the best interest of their Patron to allow their servants to meddle with other realms; for Clerics, that means that the Gods are also loathe to allow their servants to start jumping around the planes; and I'm sure you get the idea.

As for the maintenance of the status quo, again I'm not 100% sure what you mean. The group of mages that originally banished the Celestials and Fiends are long, LONG dead, and don't maintain the banishment in any personal way. The plane was created, and they were confined there, but the mages aren't somehow immortal and maintaining the spell, it was simply cast. Active Gods is something I really like to avoid, and I agree with you that there should never be super-powerful beings whose responsibility something is, making players feel pointless; instead, most if not all the Big Guys from the past are dead or missing or trapped or whatever, depending on the situation, and players are usually just the people who are having to pick up the pieces. As for the banishment failing... That's a campaign idea for another time, I think, perhaps with a high-level party.

When it comes to the issue of "What happens when a mortal dies": A soul is intangible, and as such cannot be hit with a stick. That said, one absolutely can, in theory, travel to the Realm of Shadow and ask a soul questions. Trapping the soul would be more difficult, and might require a homebrewed spell, but in theory it would be possible - if one was in the Realm of Shadow right when someone died right next to them, then yes, they could interact with that soul. A soul can be destroyed, but I confess I'm not sure what someone would have to do to destroy a soul. When an angel dies, nothing happens, it's simply dead, and the same with a demon. Nothing will happen to the mortal, or the counterpart of that outsider, it simply goes into oblivion and ceases to exist. No afterlife for them.

I think I've addressed all the concerns, but I've been writing for an hour now and my head hurts so I might be missing some.

[Rift_Wolf]

I like the idea of the River to the afterlife; why not expand it? Fiends and Celestials could be Spirits that surround the river; both offer safe passage (rather than swimming the waters or spending an eternity adrift.), but fiends offer it to the corrupt, Celestials offer it to the good. The idea of having to prepare for the journey would appear in mortal mythology; kings and Heroes would be buried in ships so they were assured safe Passage without outsider interference. The wicked might be buried with a weight around their neck, so their journey is deliberately slow and painful. Searching for the farmer who just died would be difficult; the waters teem with the dead, and there's no rhyme or reason to where a soul might be once it leaves it's mortal body. Tracing the ship of a ruler, or contacting an Angel who ferries those Fallen in battle, could be interesting story elements.

[arrowed]

Death is a river in the Sabriel-Lirael-Abhorsen series by Garth Nix. They might give inspiration if you haven't read them already.

[Scarey Nerd]

I like the idea of the River to the afterlife; why not expand it? Fiends and Celestials could be Spirits that surround the river; both offer safe passage (rather than swimming the waters or spending an eternity adrift.), but fiends offer it to the corrupt, Celestials offer it to the good. The idea of having to prepare for the journey would appear in mortal mythology; kings and Heroes would be buried in ships so they were assured safe Passage without outsider interference. The wicked might be buried with a weight around their neck, so their journey is deliberately slow and painful. Searching for the farmer who just died would be difficult; the waters teem with the dead, and there's no rhyme or reason to where a soul might be once it leaves it's mortal body. Tracing the ship of a ruler, or contacting an Angel who ferries those Fallen in battle, could be interesting story elements.

I really, really like this concept, except for one crucial thing - I wouldn't want all cultures to end up having a nautical theme to their funerary rituals, which I fear would almost certainly become the case in this scenario. However, I think you might be on to something with the idea of Angels/Celestials and Demons/Fiends offering passage through Death - perhaps instead of a river, simply a place of fog and mist whereby you can walk and walk and never find the end, unless guided? It doesn't have the same feel to it, though, so I'm not sure. I'll think on it.

Death is a river in the Sabriel-Lirael-Abhorsen series by Garth Nix. They might give inspiration if you haven't read them already.

Trust me, the difficulty is not ripping those books off TOO much. Even the campaign setting's name comes from BeliSAERE.

[arrowed]

If this is a world with typical D&D magic users who can cast Plane Shift, is there a floating city built for spirits to wait for their loved ones? Are there mortal settlements with open portals to the necropolis, used for grieving relatives to bid farewell to the dead, or for murdered citizens to name their murderers? Are there ferrymen (and ferrywomen) who row spirits more safely down the river in exchange for passing messages on to the passed-on-dead? When a dragon dies, does it fly over the river?
... I might be getting a little sidetracked. Garth Nix is awesome!

[LudicSavant]

... I might be getting a little sidetracked.

No, those are perfectly valid followup questions, the likes of which should absolutely be answered if the river of souls thing is going to come off as being something more than just being tacked on to fill a checkbox.

[ace rooster]

With regards to Angels and Demons, I'm not sure what you mean by weaponisation of Outsiders by mortals - I'm going to assume you mean mortals summoning outsiders for their own purposes, like a cult summoning a demon to unleash upon their enemies. While the DMG has spells for Summoning, Calling, Planar Travel etc, and the players can absolutely use those spells as they need, in practice the wider world is for the most part not privy to that magic. For Wizards, that means that those with the spell in their spellbook are rare in the extreme; for Sorcerers, that means that those spells only rarely awaken in Sorcerers; for Warlocks, that means it is usually not in the best interest of their Patron to allow their servants to meddle with other realms; for Clerics, that means that the Gods are also loathe to allow their servants to start jumping around the planes; and I'm sure you get the idea.

As for the maintenance of the status quo, again I'm not 100% sure what you mean. The group of mages that originally banished the Celestials and Fiends are long, LONG dead, and don't maintain the banishment in any personal way. The plane was created, and they were confined there, but the mages aren't somehow immortal and maintaining the spell, it was simply cast. Active Gods is something I really like to avoid, and I agree with you that there should never be super-powerful beings whose responsibility something is, making players feel pointless; instead, most if not all the Big Guys from the past are dead or missing or trapped or whatever, depending on the situation, and players are usually just the people who are having to pick up the pieces. As for the banishment failing... That's a campaign idea for another time, I think, perhaps with a high-level party.

Now that the mages are gone, what stops the celestial war from spilling over back onto the material. You might not have many wizards with binding or plane shift, but it only takes one to start the trade of material plane resources (spellcasting, as well as stuff, or even influence on the material) for outsider help on the material. Outsiders are very powerful compared to mortal soldiers, as well as having great utility. This trade aids the outsiders, so some outsiders would be keen to spread the spell. "This magic is rare" is not a good explanation, when there are forces that will benefit from this magic not being rare and can endevour to make it common. The outsiders will fight for influence in the material to secure this trade, even if they find it difficult to actually be present, and this will escalate to conflict on the material.

It is a bit like using the explanation "because only a few people know how to build boats" to explain why two regions are not trading. It will work for a while, but the ability to build boats will spread because it is so profitable. Worse than that, your initial explanation for why they are seperate is analogous to "they were all shipped over"... Seeing the issue?

I think we need a few more details about the banishment effect to resolve it. The alternative is have the material plane be fairly insignificant, and hence not worth the attention of celestial/demonic powers, but this doesn't sound like your setup.

[JCDread]

Presumably the spell would also be very rare in the celestial realms, or even impossible to do from their end of the realms, meaning the Angels and Demons could never cross over unless they are summoned from the other side. Also I'm not sure what magic system you are using but plane shift is a 7th level spell in 5e and most similar materials. So unless their are like a massive number of 13th level wizards and clerics running around your campaign this won't be a huge issue. Our friend from the previous comment is correct in the fact that the need to actually tip their war into a place where they can win it will be a strong drive for Angels and Demons both to move on over into the material plane and utilize it as a place to retreat to if nothing else, and if possible a place to acquire magics and weapons that they do not possess, like dragons or some other massively powerful non planar creatures as slaves, allies or anything in between. However if you are running a game where magic isn't super prevalent then perhaps the material plane doesn't have anything the Demons, Angels and other inhabitants actually want, only coming to the material plane when summoned, simply seeing the neutral part of their souls as irrelevant to their greater alignment based goals. As far as nigh immortals who are literal personifications of good and evil, their opposite personifications are of a higher concern, to them than whatever this current generation of weird meat creatures want this particular outsider to do. Also since Demons and Angels are exist largely independent of the power source gods are created from, and presumably immortalish (minus y'know actually slicing them to death,) Demons, Angels and anything else like that running around on the material plane probably rubs anything even closely related to a god, the wrong way.

[JCDread]

Also what sort of gods do you have in this particular setting?
Because to me those are probably more immediately concerning to this setting. While I fall into the more active gods camp because gods create all kinds of problems, especially if they all fall in the grey area as far as morality functions because gods can cause all kinds of messes that the PC's can solve, with an added bonus that the big bad cannot actually be killed (unless you the DM want them to be of course.) The idea that gods are created by people as opposed to the other way around creates a lot of other interesting plot and potentially story points, that can create great characters and stories. I would also like to throw in the idea that different gods probably share domains and power sets since, in theory similar cultures can in fact create similar gods, or gods of similar things, without knowing that they had inadvertently stepped on the toes of another culture's god. An interesting idea is that gods are only created with personalities, not actual power over an element, or facet of reality, simply being known to grant aid in that particular area as opposed to being the personification or ruler of that part of reality.

For example; Grock the great Orc god is a fearsome warrior and as such for the sacrifice of a defeated enemy executed on a battlefield Grock will grant a warrior strength.

Lowen Kahn the great lion headed god is also a great warrior and will also grant warriors strength in battle in exchange for a wild boar to be sacrificed and a singing of his war song (which is of course the rains of Castamere.)
neither of them are restricted to granting strength in battle this is simply what most people ask them for since both gods are known as mighty warriors.
Ashara the goddess with a thousand lovers could grant your the same blessing but maybe in return for bringing her a virgin to add to her harem of a thousand lovers (one has recently died of old age and she needs a replacement.) None of them are actually restricted on who or what they help with, its just who you remember to ask for help.

Also gods who need worshippers, not only to make them more powerful, but to survive are a very bad fit for a world full of as you call them non active or non interventionist gods. Gods who need and heavily benefit from more people knowing about them would probably spend a ridiculous amount of time actively interfering with the world to make sure people knew they were there. so if you want to keep that idea remember that logically speaking, unless the gods are ridiculously underpowered and limited in their scope and geographic area, the gods are provided with a giant incentive to interfere as actively as possible in this particular world.

[ace rooster]

Presumably the spell would also be very rare in the celestial realms, or even impossible to do from their end of the realms, meaning the Angels and Demons could never cross over unless they are summoned from the other side. Also I'm not sure what magic system you are using but plane shift is a 7th level spell in 5e and most similar materials. So unless their are like a massive number of 13th level wizards and clerics running around your campaign this won't be a huge issue. Our friend from the previous comment is correct in the fact that the need to actually tip their war into a place where they can win it will be a strong drive for Angels and Demons both to move on over into the material plane and utilize it as a place to retreat to if nothing else, and if possible a place to acquire magics and weapons that they do not possess, like dragons or some other massively powerful non planar creatures as slaves, allies or anything in between. However if you are running a game where magic isn't super prevalent then perhaps the material plane doesn't have anything the Demons, Angels and other inhabitants actually want, only coming to the material plane when summoned, simply seeing the neutral part of their souls as irrelevant to their greater alignment based goals. As far as nigh immortals who are literal personifications of good and evil, their opposite personifications are of a higher concern, to them than whatever this current generation of weird meat creatures want this particular outsider to do. Also since Demons and Angels are exist largely independent of the power source gods are created from, and presumably immortalish (minus y'know actually slicing them to death,) Demons, Angels and anything else like that running around on the material plane probably rubs anything even closely related to a god, the wrong way.

Lesser planar binding is 5th level in 3.5, though demonologists can get it as a 3rd level spell (at CL3 no less). This is a setting where cosmology altering magics have been done by mortals, so it would not make sense to me for mid level casters to be massively rare. Planar binding does not offer the retreat option, but it is easily weaponised if outsiders don't have any way to punish abuse. "Serve me for 1 day per caster level and I will trap and destroy a demon of similar power." is a pretty powerful bribe for something that's whole purpose is destroying demons. It will piss off the demons something fierce, but if they have no way of actually dealing with you that doesn't matter. If they do have some way of dealing with you, then the armies of heaven mobilise similar resources to defend their demon genocide machine, and the celestial war comes to the material in a big way.

All powerful gods have problems with player agency, but I suppose that the churches could have a crusade against demonology for precisely these reasons. It gives you a setting that needs powerful cooperative churches I think, and is suspicious of arcane magic. Could be cool.

[WantLearn]

If keeping angels and fiends is your problem, my recommendation is to make a minor adjustment to the planar summoning and transport spells. Add a material component to those spells and make that component very limited and controlled.

For example, the mages knew that eventually the fiends and angels would find a way to break the planar shield spell so they left in a well known loop hole: in order to travel between the material and angelic/fiend planes you need a piece of the mages to cast the spell. The fiends and angels know they can break the barrier, but it would take a large amount of energy and resources to do. The aftermath of breaking the barrier would leave the side that broke the barrier open to attack from the other, or if the material plane is the goal then the other side would have the extra resources to easily gain dominance on the plane. The material component for divine/fiendish summoning being rare, controlled and finite would give all kinds of plot and adventure hooks and would keep the angels and fiends out of the material plane without having to curtail other forms of planar summoning.

[Grek]

A point I think a lot of people miss with regard to 5e's advise is that while you DO need a place for ethereal creatures to exist, a place for outsiders to come from, an answer for what happens to souls in the afterlife, etc... those planes can seriously be combined into a single place.

Is there any particular reason why the Ætherium and the Crossroads are a different place? Why The Realms of the Gods aren't simply important locations in Heaven and Hell? Why the demiplanes of the Twilight Lands aren't simply regions accessible via the Crossroads?

A much more reduced cosmology would have:

The Mortal World, where the main campaign occurs.
The Ætherium, a dream-like realm filled with demiplanar bubbles, mysterious portals and winding staircases. Fey live in the bubbles and can occasionally be found wandering the staircases, and the few natural portals in and out shift with every moon.
The Afterlife, which is a foggy and wraith-haunted place where angels and demons in angelic guise guide souls away to celestial paradises and fiendish prisons hidden in the endless mists. Necromancy calls forth lost and stubborn souls who refuse to follow either.