The animated dead are the bodies of formerly living creatures animated through fell magic. Undead can be created intentionally by a necromancer to serve him, or arise naturally from the dark energies of evil regions.
 What is an undead?
An undead may be formed of either the rotting husk or the bones and shell of a being. The former is considered a zombie, and the latter a skeleton. Although vampires are no longer performing the bodily functions of a living being, they are not considered the "animated dead", this term being reserved for a corpse which has begun to move and act on its own or by the will of another, but lacking any form of intelligence beyond a primitive urge to hunt and kill any living thing it can find. Where zombies and skeletons cannot think or behave in any sophisticated manner beyond "make everything die", vampires are willful beings, generally indistinguishable from living persons (and capable of great deception to ensure that nobody learns about their condition). Ghosts are called Undead in-game, but they are also not considered animated dead (as they lack a corporeal form).
As long as the remains of a creature contain a body part capable of grasping, be it a hand or head or the entire upper half, those remains can be animated. This can lead to animated hands and heads, which seems comical until you consider the implications of a swarm of such monstrosities and the havoc that they might wreak. Currently, even some parts of creatures which should be incapable of autonomous movement can be raised, such as the hair or skin. They are, however, predictably nonlethal, mostly serving as a B-movie terror monster to scare your dwarves into running around.. A body part can be resurrected as a zombie even if it has already done so and been de-animated again. It is important to realize that an endless horde of the undead may really be endless unless the root of the problem is destroyed, or in the case of evil regions, avoided entirely.
Undead retain the wounds that killed them in life, as well as any they have sustained since or from a temporary de-animation. Undead vary in levels of strength depending on their form. Certain types of animal are likely the most dangerous that it is common to encounter, and can have dangerous strength, speed, aggression, and piercing attacks. The undead of butcherable creatures can still be butchered once de-animated, as long as they have not rotted; doing so will prevent them from re-animating again.
Larger undead with the BUILDINGDESTROYER tag can still destroy buildings, though undead with special attacks like webbing will not be able to use them. Undead thieves can still pick locks, but will not path to a locked door unless in pursuit of the living. If found underground, undead will usually path into a fort if they can.
 Thralls, Husks, and Zombies
Certain kinds of evil weather can instantly turn any syndrome-vulnerable creature into a bloodthirsty undead killer, opposed to all life. These creatures are referred to be the sort of weather that transformed them, an identifier as a thrall, husk, or zombie, and their original creature name-- for example, a stray guineahen unholy gloom husk. The specific syndromes that generate these creatures are created at the time the world is generated, but vary only slightly from one another. Some traits these creatures are likely to possess include:
- Increased strength, toughness, and/or reduced speed.
- Opposition to life, lack of emotion
- Undead status (NOT_LIVING), sterility, no attribute rust or gain
- An almost complete invulnerability to the effects of damage
Because of the plethora of tags added to these creatures, they are vulnerable only to beheading or bisection. Because the interaction can happen without first killing the target, thrall-like creatures retain any armor or weapons they were carrying. And, perhaps worst of all, they may still be contaminated with the material leading to the transformation, infecting those with whom they wrestle.
Different strategies are required to beat animated corpses and thralls, respectively. Animated corpses are not difficult to destroy, but require unconventional tactics. These undead have a hit point based damage system, and as such tend to collapse after a few hits. Blunt weapons, being less likely to sever off parts, are the preferred weapons to use. Axes and swords can cut apart the physical form of the undead; this, however, may be dangerous if the source of the undead is still active and present, as the more body parts are about, the more fodder for animation is present. In this case, it is wiser to either butcher the corpses (if they can be butchered), throw them into magma, or pulverise them with a drawbridge, which will destroy the bodies so thoroughly that they cannot be reanimated again. It should be noted that magma will not currently kill a zombie itself, however.
Undead animals can be disposed of by cage-trapping them and trading them away to passing merchants.
Evil weather thralls are much more difficult to destroy. You would not wish to attempt to kill them with puncture wounds, as their organs no longer serve them in any useful regard. Likewise, choking is ineffective against their lack of breath, and unlike corpses, blunt weapons will have next to no effect. The only way to kill thralls in combat is decapitation and bisection, therefore axes and swords should be used. If possible, and if no risk of infection is at hand, they should ideally be outnumbered by a ratio of at least two soldiers to one, as even a small wolf husk can easily bring down one armoured dwarf.
The problem of confronting thralls directly is complicated by many factors: among them are their retainment of skills and equipment, and greatly increased strength and durability as compared to animated corpses. If the responsible thralling cloud is in dust form, there is in addition a great risk of infection, creating more thralls from soldier dwarves. Any thrall carrying a melee weapon or armour can dispatch a full squad in short order even with average combat skills, making direct confrontation an unwise choice. Furthermore, due to a bug, it is not unheard of for dwarves to fight very small thralls for so long that they starve to death. Worst of all is the possibility that the thrall is still contaminated with whatever substance transformed it; if this is the case, any dwarves sent to fight the thrall will become thralls themselves if the thrall tries to wrestle them. From there, the new thralls might spread the contaminant further still, which can easily lead to a full-fledged zombie apocalypse. In these and other dire cases, it is often better to not fight them directly at all, instead resorting to traps, atom smashers and other indirect ways to neutralise them. Magma doesn't kill thralls, so caged ones might require a creative way of destroying them, especially if they can destroy buildings.
Reports have been made of zombies animated by the ambient evil of a region deanimating on their own when wandering away from such a vile place.
 Undead Fun Facts
- Undead can animate from hauled corpses.
- Undead will not attack vampires.
- Undead risen from starved animals in cages are not caged.
- Undead attack Megabeasts as well as invaders, except necromancers (as their sorcery allows them to control undead easily).
- Creatures capable of evading traps or bypassing locked doors retain that ability as undead.
- Enthralled dwarves from your fortress are not affected by traps.
- Dwarves who like an animal will also enjoy that undead animal.
Dwarves will not report someone missing as dead even if the corpse is gnawing on their ear.
Magma won't melt undead which makes evil biomes extremely difficult (and More fun). A workaround exists  by adding a boiling point to body parts in the raws.
Animated creatures can somehow become stuck in mid-air, and will not move at all, even if killed. Spatters of blood from the animated party also float in this manner. This has been known to occur in cases ranging from a deer's animated head to a goblin's partial skeleton.