Undead

An undead is a once living creature who has returned with a semblance of life, but who's body and/or soul is now absent or highly dysfunctional.

Three Components of Sapient Life
Every sentient creature is made up of a body, a soul, and a mind. Non-sentient creatures have a body and soul, but lack a full mind.

The body regulates bodily functions, physical activity, and muscle memory.

The soul maintains personality, emotions, and deeply held values.

The mind retains facts such as one's name, and anything they may have read. It also allows one to use logic and reasoning.

The Mellifer's Curse
The corpse of any sentient creature eventually rises as a zombie. The time this takes varies greatly, it usually takes between 10 and 30 days after death but it could take anywhere from mere seconds to several decades. Dead animals do not rise, unless they were granted sentience prior to death.

Different cultures have different explanations for this phenomenon. In Kippel it is believed to be a curse on the Nexus from The Mellifer, the Lich God. The only way to prevent someone's body from rising is to destroy it completely, this is usually accomplished via cremation.

Especially quick risings are most often due to the presence of evil magical energies, such as those generated from necromancy or fiendish rituals. It is a common old wives' tale that someone bitten by a zombie will turn into one. This story is meant to keep children from wandering into the woods on their own, but like most old wives' tales there is a kernel of truth to it. A person killed by a zombie will often rise faster than usual due to residual necromantic energy seeping into their wounds.

Especially slow risings are usually the result of holy preserving magics, such as those that might gather at a place of worship. These are often employed following the death of a famous warrior, artist, or well-liked leader. This gives time for the person's many mourners to say goodbye without running the risk of seeing the corpse rot or zombify.

Zombies
Dysfunctional bodies that lack a mind or soul.

The most common form of undead by far. Very few commoners will ever see any other kind of undead, but almost everyone will see a zombie at some point in their life. Generally non-hostile, but it is still advised for those not adept at combat to keep a distance due to their unpredictability. Zombies will generally lumber about the area they died.

Sometimes zombies will resume what they were doing when they died, albeit much less effectively. This can be dangerous if they were killed while fighting, as they may attack whatever comes near them. In most cases this will be unnerving but ultimately harmless activities, like gathering plants or splitting wood.

Zombies may also just walk in a single direction until their legs break down. When they reach an obstacle, they are more prone to simply walk into it as opposed to pathing around it. Because of this it is relatively common to find zombies in ponds and pits.

Much to the distress of the deceased's family, some zombies return to their routine. A wife whose husband has been missing for over a month may come down for breakfast one day and find her husband's rotted corpse sitting in his chair.

Zombie Thralls
While they are usually identical in appearance, and also lack a mind or soul, zombie thralls behave very differently from ordinary zombies. A zombie thrall is a corpse that has been raised in service to a necromancer, as opposed to being raised "naturally" by the Mellifer's Curse. Thralls never act without being explicitly commanded by their necromancer, as such their behavior is entirely dependent on the nature of the magician that raised them. Another trait unique to thralls is that they decay much more slowly than normal, whereas natural zombies decay at the same rate as a motionless corpse. Natural zombies can, and are, turned into thralls. Likewise, fallen thralls will rise again as natural zombies, usually within a couple days.

Thralls infused with especially potent necromantic energies may be risen into a more powerful form, such as a ghoul. These kinds of undead will often remain violent even after being broken from their necromancer's influence.

Skeletons
The result of either a zombie's flesh rotting away, or a corpse rising after all of its flesh and organs have been removed. These undead act in much the same way as zombies, and can also be enthralled to a necromancer. Interestingly, removing the flesh makes skeletons more capable of fine motor skills and they have even been observed using archery to great effect.

Ghosts
Ghosts are purely souls, and lack a body or mind.

Most ghosts lack any memory of their lives but retain their personality. It is unclear why ghosts appear as opposed to having their souls move onto the Heavens or Abyss. Prevailing theories are that they died before finishing an important goal, or they were never given proper funeral rites after they died.