Planes and Deities
Despite the title of this post, there are no deities of the traditional D&D type in this setting. However, there are numerous beings that fill the role that deities do in other settings, so I'll present them under that title.
The 3rd Ed. Manual of the Planes was one of my, if not the, first supplement books I got for D&D, and it's still one of my favorites. Probably because it's extremely fluffy and even the crunch in it also elaborates on the fluff. At an earlier point I wanted to use the Great Wheel, then tried to modify and crop it down a bit, until I finally ended up with only 5 planes left. If you're not planning to do a lot of plane walking and having a planar home for each of your 200 deities, you can really do with a lot less planes and concentrating on a small number of well developed aspects is a major part of the design concept for this setting. But still, I used the MotP extensively, and I think it shows.
(I think I split this into two posts. When this one is posted, the second part is still currently being written and will show up shortly.)
The Realms of Existence
The basic layout of the planes is quite simple. You have the Realm of Mortals, which is the usual prime material plane, and the Spiritworld which is kind of a highly magical reflection of the material plane. Third is the Realm of Shadows, which resembles as dark and lifeless reflection of the material plane. Surrounding this core of three planes is the Void Beyond the Realms, which fills the role of the Astral Plane, but also stands in for the Outer Planes and the Far Realm.
A special case is the Underworld, a fifth realm that in terms of the MotP and the DMG would probably be a second layer of the material plane, but more on that in the planes own entry.
The Realm of Mortals
The Realm of Mortals is the material plane of the Ancient Lands setting and mostly follows all the usual conventions. While it seems to most people as the "normal" world, it is however only one half of the world of which the Spiritworld is the other half.
The mortal realm is the the physical half of the world and almost entirely made out of matter. Humanoids and animals are the exception, as they also have a soul or a spirit, which the rest of the mortal realm lacks. All magic actually originates from the Spiritworld (or the Void, but more on that later) and creatures from the mortal realm usually have no natural magical ailities. Mages are the exception here, but even they don't have magical powers as a natural ability of their humanoid nature.
The mortal realm works the same way as our own in almost every other respect. The Ancient Lands are located on a planet slightly smaller than Earth but with a slightly higher percentage of water covering the surface. Travel by sea is an important aspect of life for people in most regions and the arts of shipbuilding and navigation are far more advanced than one would usually expect in a late bronze-age/early iron-age setting. Strong winds blow almost on every point of the world for most of the year, so it's no supprise that the spirits of the wind have a high status in many cultures.
The other half of the world is the Spiritworld. Where the mortal realm is the realm of matter and physical objects, the Spiritworld is the realm of life force, spirit energy, and magic. While on first sight the Spiritworld seems not that different from the mortal realm, the environment is mostly spiritual energy given a solid shape. For everything that exists as part of the natural world in the realm of mortals, a spirit counterpart exists in the Spiritworld. They two parts of each pair may not always be in exactly the same shape and trees and mountains may be in a slightly different position in relation to each other in the Spiritworld, but there are always two of each. The exception are humanoids and animals, which have their spirit or soul already residing inside their physical bodies in the mortal realm. Similarly, monstrous humanoids and magical beasts come into existance with a physical body in the Spiritworld and don't have a counterpart in the mortal realm. However, being creatures of the spiritworld, they usually have many magical abilities, just like other spirits do. For most trees or rocks, their counterpart in the Spiritworld will also seem like nothing like a tree or rock. But certain special places and objects have extremely powerful spirits which are known as demigods. Spirits will be described in more detail later on.
The Spiritworld resmelbes the mortal realm in most aspects, but being a world of spiritual energies and natural life force, everything is taller, stronger, and more powerful. Mountains are higher, storms blow stronger and are usually accompanied by heavy lightning, the cold of winter can freeze people solid and in summer the sun can burn unprotected skin within minutes. While it looks amazing at first, the spiritworld is actually very hostile to mortals and only seasoned adventurer survive there for more than a day. Generally speaking, the Spiritworld is even more of an untouched wilderness than the mortal realm already is. The castles of monstrous humanoids and their demigod lords lie just in the middle of the forests or atop the mountains, with no actual towns and cities, or any roads connecting them. Some villages exist, but they are generally small and don't offer much for visitors from the mortal realm. Most of the people are monstrous humanoids like oni, minotaurs, naga, shee (eladrin/half-celestial elves), and aranea.
Special Features of the Spiritworld
Time in the Spiritworld: within the Spiritworld, time seems to flow slower (or faster, depending how you look at it) than in the mortal realm. While one day in the mortal realm equals one day in the Spiritworld, in the Spiritworld it seems to last about twice as long. A day seems to have about 48 hours, but even humanoids don't seem to need more food or rest per day, than they would during a day in the mortal realm. But when they return to their own world, the sun has set and risen no more often than it did in the spiritworld. The effect becomes even stronger when one stays in the spiritworld for longer periods of time. Aging and errosion seems to happen at only one tenth of the normal rate for the mortal realm. People can have left the mortal realm for decades, but when they return they still have aged by only a few years and abandoned ruins in the spiritworld remain in a very good shape for far much longer than they would in the mortal realm.
Magic in the Spiritworld: The spiritworld is the origin of all magical energies and all spells cast there are much more powerful than in the mortal realm. All spells and spell-like abilities cast in the Spiritworld are affected as by the Empower Spell feat without any changes to the required spell slot or casting time.
The Realm of Shadows
The Realm of Shadows is basically a Plane of Shadows, but also with the Ethereal Plane thrown in.
It is a dark place almost entirely made out of shadows and inhabited by only a very few shadow spirits. Few people go there and most do so to make use of it's unusual nature that travel in the Realm of Shadow brings you much faster to your destination than in the mortal realm. However entering the realm of shadow is not easy and travel there is combined with many dangers. But still, there's some trafic in the Realm of Shadow and reportedly even some settlements used as well hidden havens by those who frequently find themselves in this bleak place.
The Underworld resembles the realm of mortals in almost all aspect, except that it lies entirely underground and has no known surface. The underworld is reached by portals found in the deepest caves and oceanig caverns of the world that are usually entirely invisible which makes it almost impossible to tell when one has crossed the border between the two realms. It is an endless system of caves and tunnels, some dry, others flooded, and visited by travelers from the mortal realm even more rarely than the Realm of Shadows. It's the home of aberrations and other stange but powerful creatures like aboleths, krakens, and doppelgangers. These creatures rarely find their way into the mortal realm, but if they do so, it's usually a great deal of trouble for everyone who crosses their way.
The Void Beyond the Realms
In addition to the other Realms of Existnce, there is also a place that lies beyond them, known to most simply as the Void. The void lies even beyond the stars and such concepts as time or distance don't seem to have much of a meaning there. Despite its name, the void is not entirely empty and it has it's own spirits, known as daemons, who often create domains that take the form of small islands of relatively normal environments, though many of them are still thousands of miles across. Some suspect that there are in fact many universes, all of them connected by the Void. This would also explain why the daemons rarely show any interest in the worlds of the Ancient Lands at all. It's just one planet in a single universe among a posibly infinite number, so there's nothing special about it in the eyes of daemons.
Long ago, some mages have discovered that the Spiritworld is not the only source of magical energies, but that the Void also does have a magical essence of its own that also can be tapped into be mortal spellcasters. These mages are known as sorcerers, who are widely mistrusted as their use of the energies of the Void often has unexpected and devastating side effects. Sorcerers are at a particular high risk of getting the attention of a daemon and almost all cases in which daemons entered the world of mortals have been the result of overconfident sorcerers.
The void itself appears to most people who have claimed to see it as an enless space of many colored starts and clouds of silvery mist at impossibly far distances from their point of view. Mortals appear to remain in theie physical forms, but this is mostly an illusion created by minds that can not comprehend a reality in which shape and location doesn't have any meaning. The domains of the daemon lords can take almost any shape imaginable, some creating some sense of familiarity, but most more bizare than the one before it. Still, they mostly seem to follow most laws of nature as in the Realm of Mortals, so they are slightly less disorientating than the open Void.