"A summary of a general understanding of undead in the world, and how they come to exist."
Necromancy is the magical art of raising and controlling undead creatures through powerful manipulation of a body, a spirit or both. More powerful necromancy may solely target a spirit and have no need for a body for its ends. Necromancy is used on targets for the necromancer to raise and control, however there are spells that target the caster themself thereby turning themselves into more powerful undead. Necromancy may be achieved through either clerical or arcane spells.
In order to have an undead creature of any kind something is required to have first died. The resulting necromantic magic creates undead: not alive, but not at rest. They never tire, never sleep, and have no life in them as any mortal would call it.
There are varying types of undead with vastly different strengths, weaknesses, and capabilities. Whereas the majority of undead are mindless servants, weak on their own but a force when in numbers, are most common to be raised and fought against. Any undead with glowing eyes is known to be extremely powerful with the ability to communicate, interact, and sometimes raise other undead.
NATURALLY OCCURING UNDEAD
A spirit that has not fully passed on from its current mortal coil, or perhaps the mortal realm, may become a wraith, a very specific type of undead. Though this can be created through the use of necromancy it is known that a restless spirit if left too long, whether it has been unable to resurrect or pass on to what comes after life, may turn into a wraith. These angry spirits lash out at all manner of living creatures wreaking havoc in their path. While wraiths are the most common naturally occuring undead, others are sometimes possible through remains found in areas of intense links to tortured spirits, echoes of powerful necromantic magic or other unsettling events.