Even the undead want something.
It’s traditionally said that spirits stick around to finish an important task or protect someone or something. This might be more of a character issue than one about an undead type, unless the latter always has the same goal that is apparent in behavior. Perhaps their appearance and behavior are often the same so that when taken together, the undead type is more identifiable. However, this is a basic idea about ghosts and is nothing new. We’ll have to combine this with more unique issues to achieve something unique.
With unfinished business, two undead are unlikely to have the same tasks in mind unless they were created at the same moment and want retribution, for example. Two independently created undead might go about a similar goal in ways so different that this cannot be used to identify them. Revenge via murder will depend so much on their intended victim that the lack of patterns would make it harder to identify this undead as a given type.
Being undead might offer someone a chance at returning to life. Logically, a spirit needs a new body to inhabit. Getting back the old one won’t do much good if it’s still dead and decayed, and yet you’ll find no shortage of stories where that exact thing happens and the body is magically (literally) restored. Technology can also be used to that effect. People don’t seem to question this, but maybe creators should. Decide whether body restoration is possible. If not, our spirit can forget about its own body. Either way we have some options, and so does our spirit. These can help us craft undead with certain capabilities that distinguish them from each other or standard ideas.
First, decide what will heal a body and don’t just gloss over this without commenting on it. Is it a potion? A priest? A device? How much healing can be done? If the body is nothing more than ashes because someone burned the body, can the spirit still get the body back? That would be extreme. Less extreme is a body that’s been dead a couple days. It arguably takes less power to restore that one.
If the body can only be restored enough to support life but is badly wounded, the spirit will need to address that, and maybe know this in advance and have someone standing by to finish healing it. This would be a standard concern for this particular undead type. Imagine people sighting this undead and realizing that it must be lining up a doctor and how this might affect their decisions to stop it.
What kind of individual would be willing to help our spirit? Family? Someone in it for money? Or being coerced by that spirit? A spirit terrorizing someone into helping carries an interesting side effect—if the undead is brought back to life, it loses its newfound powers (if it had any), and can become vulnerable to the one who has just been terrorized into restoring it. Is the undead character smart enough to foresee such an outcome and take precautions against revenge, keeping leverage that continues into its new life?
Whether our spirit’s own body is available or not, it can try to take someone else’s. There are several options to consider here. First, will the new body need to be missing its soul? In this scenario, a smart spirit that is capable of jumping into a body should hover around places where young, healthy people die, like an army in battle. But there’s still the problem of inhabiting a body that’s now dead, unless this spirit is powerful enough to force locomotion and then get someone to heal it, too. This seems impractical.
Second, if the spirit can take over a body with a soul, what happens to the existing soul? It is ousted or still there but suppressed? The former case causes another spirit to be on the loose. The latter offers an internal struggle; will our spirit be powerful or forceful enough in personality to win or end up the one suppressed but now trapped within someone else? Or can they jump out whenever they want?
An undead who gets itself restored to life faces an interesting prospect. Now what?
If they’ve got their own body (healed or not), how will people react to this? Will old acquaintances and family have some sense of what’s happened? Is this person known to have been dead and then resurrected or is he assumed to have been in an afterlife? The latter suggest a peaceful return and time prior to that. The former suggests sinister, unsettling issues. And what of all their worldly possessions, especially if they’ve been legally declared dead? Do they have options for restitution? Is our world a place where people return from the dead all the time and have certain rights they can expect? A Bill of Undead Rights is just what the world needs.
If they have a different body, what are they going to do? Embark on a new life as that person or try to reintegrate themselves into their old life as their old self? In the latter case, will anyone believe they’re who they say they are? How would people react? Has our undead done anything to prepare for such an issue? Wouldn’t it be better if a spirit attempting to do this has made plans for this while still disembodied, and that there’s a type of spirit that typically does these things? That makes this less an issue of that character and more a behavior of this undead type.
If they start a new life in the new body, might they run into someone who knew the previous owner of that body? Such a concern could have them moving far away, so have they planned for this? Would our undead be intending to start up a new life and therefore engaging in certain behaviors that are typical of this undead type? Imagine a spirit who can steal gold for their new life, and haunt a place they want to live once they have a body again, driving out the living so no one will take the home until they arrive in their new skin later, buying it with that stolen money. This can be a character, but it can also be a type of undead.
Undead are considered damned to wander for eternity and likely want to be at peace instead. The longer they’re undead, the more this could be true, and the more upset they could be. It may not know how to reach this goal. Does our undead seek priests or family members for help? It may not realize the fear it will cause or naively think help will come when an attempt at destroying it might be the result instead, though this could achieve peace of a different sort anyway.
An undead may simply want to be left alone, lying in a grave until compelled to emerge, such as vampires that need to feed. Spirits might want to exist in a perpetual state of denial that they have indeed died, haunting their homes in a dream-like state where the passage of time doesn’t register on them, as if they died this morning when decades could’ve passed. Some might want to simply retrace steps from a happier time and be content with this existence, only to have frightened people interfere with them. Such an undead would be harmless but assumed to be dangerous by those unfamiliar with this type’s traits. These are standard ideas.
In theory, everyone wants peace, but some enjoy riling up others in life. If they become undead, especially spirits that can appear and disappear at will, tormenting others is easy. This can be innocent fun or an attempt to mentally and emotionally destroy their victim(s). This can be for revenge against just one person or all humanity (or a species).