undead

The remains of things once living that have been reanimated and act on their own are commonly referred to as undead. This is a type of creature, but it can also be considered a condition in many respects.

Undead Behavior

Undead don't generally need to eat, drink, sleep or breathe, so many things that deter the living have no effect on them.

Undead intelligence

Undead are usually animated by something other than the original soul that ran the body, so what they remember of their lives and their relative intelligence varies widely depending on the type of undead. In order from least to most intelligent:

Mindless

Many of the most common undead are effectively mindless - they ignore obstacles or dangers to follow commands (or pursue the living). Zombies are the most common example of this sort of behavior.

Detached

These creatures don't fully recall their lives, but may fall into behaviors they often took in life (though it's more "going through the motions" in some cases rather than true understanding). They may take actions more complicated than mindless undead, but aren't as fully sapient as their living counterparts. Skeletons often behave in this way (though they can potentially be mindless instead).

Fully Realized

More complicated undead can have drives, make plans, and act in complex ways. This makes them as dangerous as the living (and arguably more dangerous in many respects). Liches and vampires are fair examples of this kind of behavior.

Fighting the dead

Immunites

Conditions

Several conditions are commonly ignored by the undead, most notably charmed, frightened, and poisoned.
Undead with incorporeal bodies often ignore grappled, paralyzed, petrified, prone and restrained.

Damage types

Most undead are immune to damage from poison.
Undead with incorporeal bodies often ignore nonmagical weapon attacks. Many undead take no damage from necrotic sources, as they are animated by that kind of energy.

Resistances

Piercing or slashing weapons are often less effective on creatures that lack flesh (most bony undead). Due to being animated by magic, many undead take reduced damage from nonmagical weapon attacks. Powerful undead are more resistant or immune to turning.

Vulnerabilites

Bludgeoning damage is often very effective against bony undead.
Several undead dislike sunlight (most notably vampires, but also spectres and wights) and may be adversely affected by it. This occasionally extends to taking extra damage from radiant sources, though this is comparatively rare. Many undead can be turned, but powerful undead are more resistant or immune to this effect. Undead take extra damage from a paladin's divine smite.

Clerics

Deities often empower their followers to turn undead, causing them to flee if not attacked directly. Aside from that, the spells to both create and prevent undead are usually accessible to clerics.

Paladins

Holy warriors can empower their strikes with a divine smite. While this is effective on most creatures, it is especially potent against fiends and the undead.

Transmission & Vectors

Some undead (most notably vampires) can create more of their own kind or other kinds of undead. Sometimes this can happen very quickly by killing a living creature, and in other cases it can be done with a corpse of the appropriate level of freshness or decay.

Causes

There are several things known or suspected to cause the undead to rise, in order of descending certainty:

1. The spells Animate Dead (DnD Beyond) and Create Undead (DnD Beyond), with others of their kind
2. An event wherein many people died in close proximity, especially when one or more of the conditions below are also true
3. Some upwelling of negative or otherwise death-related energy
4. Collections of dead bodies that were not "properly" handled

Treatment

Generally the undead are destroyed rather than cured.

Spells

Raise Dead (DnD Beyond) and Resurrection (DnD Beyond) don't work on an undead creature, though True Resurrection (DnD Beyond) does. Note that these spells usually require that the soul of the creature is willing to return, which is not guaranteed (especially in the case of creatures that have been dead a long time).

Other means

Other cures may be possible, depending on the specific kind of undead, but if there are any they are not widely known.

Affected Groups

Most living creatures can (in theory) become undead by some means. While there are exceptions to this rule (many extraplanar creatures return to their home plane when slain, and warforged cannot be raised as undead if the Llyrian Research is to be believed), these are not widely known or well understood.

Prevention

Several means can be employed to keep the dead from rising. Most notable:

1. The Gentle Repose (DnD Beyond) spell
2. The Ceremony (DnD Beyond) spell
3. Cremation
4. Burying the corpse in hallowed ground or under other "blessed" conditions

Cultural Reception

The undead tend to creep people out at the very least, and most undead are hostile/evil by default (though it's not clear why this is true or that it must be true in all cases).
As a result, your standard townsperson or traveler is likely to react with fear, horror, or aggression if they encounter the undead, and may react negatively to those associated with the undead.

Type

Magical

Rarity

Uncommon