Thanks for all the input. I'm reading, but it might take a while for me to process everything and type up a full reply. For now, I'll just respond to a few things. (Me, from the future: Lawl, a "few" things.)

Quote Originally Posted by Yanagi View Post
Do these undead have subsistence needs equivalent to food and water? Because if they don't, or just have far less, that's a lot of labor that doesn't have to be done, just to survive. That manpower could be switched over to something else profitably.

Shelter means buildings, and different types have different shelter needs w/r/t specific vulnerabilities, so it's likely that enclaves would be more homogeneous and cities would have districts to meet different needs. Do these undead have comfort zones or worry about the equivalent of exposure to excess heat/cold/moisture? I ask because a prominent feature of an undead community would be that they could live in large numbers in extreme-to-the-point-of-unliveable environments, like in the permafrost or the death zone atop a high mountain...which would make them safe from most things not actively seeking them out.
Undead explicitly do not require air, food, drink, or sleep, and are immune to disease and exhaustion. I believe the way D&D 5e implements extreme heat and cold is that the character suffers exhaustion on a failed saving throw unless they have fire or cold resistance, but as undead are immune to exhaustion I don't think these are a factor, either. So this would certainly suggest settling more extreme conditions, both as a defense measure and because it's less likely to be already settled.

My initial conceptualization was that the undead would live in sort of undercities, with villages of living dotting the surface at various exit points from the undercity. A traveler passing across the surface might never know that undead lived there. As aimlessPolymath points out, fresh corpses are needed to repopulate, although given that undead don't age there'd probably be very few new undead allowed, at least during peace times. Corpses could also be used to create regular mindless undead to be used as slaves or soldiers. In any case, the living vassals of the undead would probably have some sort of feudal relationship with the undead. As the undead don't require a lot of what most feudal lords do (food, living soldiers, etc.), the burden would probably be much lighter, and the benefits of protection make it a worthwhile relationship. Also, just because they don't need to eat or sleep doesn't mean they can't. I'm leaning toward saying that those without tongues aren't able to taste even if they can eat, but I also don't want to give up the potential for milk jokes with the skeletons. So it's possible that some agriculture would be used for food-as-a-luxury, since it isn't a need.

As undead are immune to disease, I could see them specifically cultivating disease around their settlements as a defensive measure. However, this makes less sense if they do have living vassals who would also be in danger. Also, even if it doesn't hurt them, that doesn't mean undead wouldn't be neat or hygienic. So biological weapons might be constructed and, if necessary, used, but it's less likely that they would turn their cities into biological hazards.

Since different undead types have different advantages, they'd likely find it easier to operate in different sectors...and if there's central planning, they'd likely be assigned.
I haven't thought a lot about the specific social systems. I do know that there would probably be some strict laws. The undead wizard who rules would be harsh, but fair. In particular, the spell to create more of these undead is heavily controlled, and only his clerics are allowed to cast it. Spellbooks or scrolls with this spell are hunted down and destroyed. One of the worst crimes is creating an undead child; as the undead don't age, they'll be stuck forever with the body and mind of a child. And a lot of undead still retain very human desires for family, so the black market for undead children is pretty active. I originally envisioned the ruler as Lawful Neutral rather than Good, so those caught facilitating the creation of undead children might have their souls taken and sold to devils (who are Lawful Evil in D&D, so one step away from Lawful Neutral), or some similarly horrible fate.

I can also see the undead becoming quite a decadent society. They don't need anything to live, and they're (reasonably) hard to kill. The only goal I can think of for them to strive for is to save up enough gold to buy a resurrection in the event they do die. In a twist of irony, I'd expect a creature who can live forever to be even more concerned about their own mortality. That said, as they were once living, it's reasonably that their former passions would carry over into undeath, so I wouldn't be surprised if many of them carried on in their former labors even when they don't need to to survive. Besides, everybody likes having stuff, and stuff requires money, and money requires working.

So the image taking shape is a society that appears strict and ordered on the surface, but with a seedy and decadent underbelly. In a way, it kind of sounds a lot like the Imperium from Warhammer 40k, except probably with less planet-wide genocides because some chucklefudge chaos cultist decided to summon a demon. A mix of those fanatically loyal to the ruler, those just trying to make an honest living (unliving?), and those who are the most disgusting pieces of filth engaging in every vice known and unknown.

The phatomarions' ability is so useful it would likely be considered a vital asset to the national welfare, to the point that I could see it being kept under wraps and never admitted to outsiders.
This idea intrigues me. An entire subrace of undead that is basically kept under wraps as a state secret. Do even other undead know? It would be kind of hard to hide it, and potentially too useful to keep it a secret from your own subjects. Their abilities, used correctly, definitely have a lot of potential. One thing I should probably clarify, as far as combat goes, if the body/object they're possessing takes damage, so do they. So if the body they're possessing is destroyed, they might not be in good enough shape to simply possess another object and keep fighting, and they might even go down before their body does. I felt this was necessary to balance it for players. That said, I've been thinking about the potential for things like possessing catapults and other siege engines or large machines, and there's definitely a lot of potential on the battlefield.

Quote Originally Posted by aimlessPolymath View Post
ex. skulgrims stuffing themselves with cushioning material packing-peanut-style to reduce fall damage,
I chuckled at this.

cities afflicted by plague,
Given that they're immune to disease, they would be especially suited for dealing with plagues. This could be a great asset to their living neighbors.

etc, as well as offering their services putting down unintelligent undead. This last is additionally relevant to help them distinguish their reputation from "regular" undead.
I know different settings treat necromancy differently. The default D&D position seems to be that raising an undead is always evil, but never really explains why. I could understand if it did something to that person's soul. I generally opt for a more neutral and utilitarian perspective of necromancy; that person is dead and doesn't need the body anymore, and what you're creating is basically a robot so it can't really be abused or mistreated. They're the perfect slaves and soldiers. So I can see them making heavy use of mindless undead. I can even see their living vassals making arrangements where they sell their bodies on death in order to benefit their family, sort of like life insurance.

That said, there are other undead out there, some mindless, some not, and most of them are malevolent. Hunting down and exterminating these other undead that have no interest cooperating with the living would probably be a worthwhile endeavor for them to pursue. Given their precarious position, anything they can do to improve their reputation among the living is helpful.

It's not clear to me whether their limbs are fully replaceable (ex. if they lose a hand, can they get another?), but that's a relatively small factor. They may be some of the most rapidly-growing caste, because they can be constructed from "spare parts"; parts from animal corpses may be a major contributor to their population. They're also natural fits for tailors if they have practice stitching themselves up.
By default, both fleshdolls and skulgrim can only reattach their own body parts, they can't replace them with parts from another creature. That said, since there are so many cool undead abilities, I made up a bunch of "racial" feats for undead PCs. One allows them to regenerate missing body parts. Another allows them to attach parts from other creatures, including adding new parts like extra limbs. While these are intended mostly for players, it's not unlikely that some NPC undead might have one of these special feats. That said, they're mostly reserved for the stronger undead.

I hadn't quite thought through the implications of making them from spare parts, or incorporating animal parts. As the inspiration is (obviously) Frankenstein's monster, I probably should have seen that coming. Those made from (mostly) a single corpse might be more or less stable, while those who are a hodgepodge of different corpses might suffer from multiple personalities. Or maybe they gain an entirely new personality, which would make them distinct from all the other kinds of undead.

Phantomarions are pretty great and can do a whole lot of things.
I didn't think I could make something that was balanced for a player, but I'm pretty pleased with the concept. Being able to fly and go through walls would normally be laughably overpowered for a PC, but I think the restrictions on them keep it from getting too crazy while still allowing them to do some cool things. The possession mechanic will be pretty interesting to use, too, and I think it will give players a lot of incentive to use it instead of staying incorporeal all the time. That said, I still need to hammer out the specific mechanics. Although they share going through walls with the shadowlurks, the possession ability adds a whole new level to what they could do in the undead society, so it will be interesting to think of how that can be used.

Speaking of shadowlurks, while the phantomarions are unable to carry their lichstones through walls (they can only pass through walls while incorporeal, and can't carry anything while incorporeal) a shadowlurk could, so by working together the phantomarion could get a lot more range. I feel like there's a buddy cop show there.

Alright, it's 4am, I better stop for now.