Undead are either the spirits of the dead or animate corpses - or both at once. In either case, undead are generally regarded as deeply unnatural - something that should have moved on to a different world (or stay in the ground), but refused to do so.
The undead may be sapient or non-sapient and may or may not possess a physical body.
Why someone is wandering about undead rather than rotting in peace varies - often by the type of undead. Some of them are unable to rest due to the nature of their death - hunger, pain, frustration or rage at the time of death may bring them back, as might a desire for revenge or the completion of unfinished work. Various Asian traditions doom those who die untimely deaths to existence as 'hungry ghosts' until they can find someone to take their place. They might also rise because of a failure of correct burial rites (possibly because of how and where they died) or the neglect or desecration of their tomb.
Alternatively, the undead may be created by unholy magic - often called necromancy - which can summon and bind the souls of the dead, forcing them back into their bodies (or preventing them from leaving at the point of death) to animate them. On occasion, evil magicians may even do this to themselves as a way of avoiding death. As an alternative to recalling the dead person's own soul, the necromancer may call up a different spirit and allow/persuade/force it to occupy and animate the body1. Particularly powerful evils may even animate the dead accidentally or promiscuously, leading to an epidemic of free roaming corpses.
Undeath may also be a curse, visited on a wicked or impious person for their crimes - often desecration of holy places or cannibalism. Since it must be assumed that the 'good' deities who would normally punish such offences are unlikely to approve of undead, the actual curse can probably be assumed to consist of handing over/abandoning the offender's soul to some tormentor.
Very occasionally undead creatures may be less obnoxious and be on relatively good terms with the mortal community as a form of living ancestor - indeed they may even be subject to some degree of veneration. Such undead are generally not prone to harming the living in any way, shape or form (or are very subtle about it if they do2).
At least partially due to this variety there is a great deal of variation in the mentality and behaviour of the undead - some are mindlessly aggressive, some bound in endless patterns of compulsive behaviour, some helpless slaves of their creator and some self aware and free-willed to some degree, however hedged about that may be with whatever obsession stands between them and a proper death. Some of them may not even realise that they have died. Quite often their original personality will have been twisted out of shape by being undead (if it is still the same person using the body) - a common trope is that the undead may remember having intense emotions towards someone, but can no longer express these in a positive manner…
Typically undead have some fairly significant immunities (poison, disease, suffocation/drowning, heat (short of fire) and cold (short of freezing solid)) and are less vulnerable to some kinds of weapon and many kinds of magic. Some kinds may be seen to eat and/or drink, but they can almost never be starved in any useful manner. Against that, they will normally have various banes (think of vampires and garlic), be more vulnerable to other kinds of attack (including some kinds of magic) and repulsed or destroyed by "holy" things. Incorporeal undead (those without bodies) are typically immune to (non-magical) weapons and able to move through solid objects, making them a menace in combat. Psychopomps may also be a great help when plagued with undead.
Undead can rarely - if ever - reproduce but some are shown to "spawn", converting their victims into something like themselves.
List of Undead
- Blue Men
- Frankenstein's Monster
- The Grim Reaper
- Skeleton (undead)
- Sluagh … assuming that they aren't fae.
- Worm that Walks
Game and Story Use
- Undead work well in horror stories because they are deeply unnatural and come together with all sorts of death-related motifs.
- Good as a source of body horror as well if the PCs take casualties … and fight them later.
- For less involved stories, they make good mooks who can be hacked to pieces without the slightest qualm of conscience.
- The spontaneous rising of undead is often alluded to in fRPGs, but very rarely integrated into either the rules or the campaign settings - a world where the PCs would need to perform some kind of funeral (or other apotropaic rites) on their dead opponents to avoid the risk of having to fight them again would be quite novel in game terms, but entirely congruent historically.
- Such rites may be a source of values dissonance if the other side regard them as corpse desecration (as, historically, British troops regarded the Zulu habit of disembowelling enemy dead to ensure that their soul could escape and wouldn't hang around to cause trouble).
- Undead (or constructs) can be used to recycle robot based plots into fantasy settings.
- The character of the undead, and their place in things, is likely to depend on how they came about - "corpse animated by evil spirit" is likely to be the kind best avoided - where the animating spirit is the corpse's former soul, things will depend on whether that soul was enslaved and forced into its role, whether it is there because of a curse (or inadequate burial rites) or because it is simply avoiding death3. A corpse animated by a purely artificial spirit is closer to a construct built of post-consumer human.
- Correct manipulation of undead creation may be possible - for example, deliberately starving someone to death, especially if they are tormented with food just beyond their reach, may generate perpetually "hungry" undead that will try to feed on anything that moves.