Dungeons and Dragons has a very good gamish way of describing it, distinguishing between body, soul, and mind. But that's simplistic. Toady, as we all know, likes to go into DETAIL. I mean, when assigning personalities, he didn't go with the simple Myers-Briggs typology with its 4 axis, but with another system with THIRTY. So we should be looking at systems with the maximum number of distinctions in souls. With more distinctions, we can have effects like selling PART of one's soul to various powers, have differing types of undead who lack different parts of the soul, and so on.
And here, I'm looking at the Jews. Basic Judaism posits 5 souls, (while kabbalah adds a couple more which should be useful). I can't think of any religion that has so many distinctions, so for this I think it may be what Dwarf Fortress needs.
We have:
- The "Mind/Body" connection. The part of your soul that grants access to your body. This part of the soul can be killed, which incidentally also kills you. If your body were to die but not your nephish, presumably you'd be able to be a ghost and possess people. (Nephish)
- Emotions. (Ruach)
- Intellect. So skills and such would be associated here. (Neshama)
- "Spirituality" / "The ability to know G-d". In Dwarf Fortress, this would be the part of the soul that you'd likely barter with various powers, who would use it to power their abilities, I'd guess. (Chayyah)
- "You". The you who is you. The one wondering, "who am I?" The "I think, therefor I am" part. In Dwarf Fortress, this part would make a creature self-actuated, and lacking it would make them unable to do anything except on command by another. (Yehidah)
- Prophesy / Magic. Clearly, not everyone has this part of the soul. Having this could be what distinguishes wizards from everyone else. (Ruach HaKodesh)
- Joy. Lacking this would make happy thoughts work, but would still allow unhappy thoughts. (Neshamah Yeseira)
- The "adult" spirit. (Neshamah Kedosha) While I don't think this translates into the game well, it's convenient to have 8 bits... so why not bit let this be the PRESENCE of a body, not just the means to control it.
So, by combining or lacking various forms of these, we can can see various forms of undead arise naturally.
Ghosts would lack a body. Zombies would lack intellect and emotion. A lich would possess magic, but lack joy. Vampire Spawn would lack themselves, making them slaves of their Vampire master. And so on and so forth. Various varieties could be easily created by just tweaking an undead possessing or not possessing various parts of the soul. And each would be creepy in its own way.