I’ve always enjoyed working out the ecology of a dungeon and adding creatures and features that suit and sustain that ecology, and I like the idea of even undead having an ecology–it adds some flavor to the monster concept.
In Five Moons RPG, living creatures have souls–the energy of life that leaves the body upon death. There are willful souls who, despite their body dying, persist in some way. and retain some aspect of their living consciousness. In refusing to accept their death, they seek out a corpse (their own or another’s) in a desperate attempt to emulate life. Most of these are too weak to animate more than a skeleton, and the process takes most of their will and intellect, leaving only a painful existence dominated by anger and violent urges. Thus, the most common undead is a skeleton who instinctively attacks other creatures.
However, by consuming other creatures*, these wretched undead can regain some of their physical power and intellect, and become closer to human–but still agonizingly inhuman and wanting more.
A skeleton lacks flesh and craves it. Once it consumes enough of the right flesh, it becomes a zombie.
A zombie has flesh but is still nearly mindless, so it craves brains; once it consumes enough of the right brains, it becomes a ghoul.
A ghoul has regained most of its human intelligence, and its flesh is no longer rotting, but its bones are still dead, so it craves marrow. Without its own marrow, it carries diseases (because its body cannot eradicate infection) and lacks vitality. Once a ghoul consumes enough of the right marrow, it becomes a vampire.
A vampire has a human-level intellect and pallid a human-like form, but its metabolism is poor at recycling its blood, so it must rely on the fresh blood of others and craves it. After feeding on fresh blood, a vampire looks and feels nearly indistinguishable from a human. Perhaps, if it consumes enough of the right blood, it becomes something greater than a vampire–such as a vampire that can walk in sunlight, or a human with full vampire powers.
Of course, there are ways to shortcut, bypass, or alter some of these steps. For example, a necromancer could empower a dead soul and bind it into a type of body that they want for it, creating a zombie instead of a skeleton. Some preservation techniques might allow a soul to bypass earlier stages, such as reanimating its own body as a mummy (which, as an intelligent rot-causing undead, bridges the gap between rotting-mindless zombies and cohesive-intelligent ghouls) or lich (a deliberate preparation of the flesh to house its own bolstered spirit after death). Unusual individual undead might luck upon the right combination of “food” and bypass a step, jumping from mindless skeleton to an intelligent skeletal champion. A vampire, nearly human in its form, leaves its victim’s corpses in a nearly-human state and ready for possession by the newly-departed soul to form a new vampire. Sometimes the process might go wrong (like a mutation or a cancer) and instead form a mohrg or wight instead of a ghoul, or a nosferatu instead of a vampire. But most unexceptional undead are on the simple path, and most never advance more than one or two steps.
(Also, just exactly what are the “right” flesh, brains, marrow, or blood might be different for each individual undead; this keeps it from being a simple process where you can feed a “pet” undead a certain number of corpses and have it transform into a more powerful type. Some undead might spend centuries searching for the “right” thing to eat, and never find it.)
If you like this post and where these ideas are going, please check out the kickstarter for my Five Moons RPG, which uses these ideas. Thanks!
* Early on in the 3E development process, the designers thought about adding a “need” concept to undead, appropriate to each type (zombies need brains, vampires need blood, and so on). But they ended up not using that idea.