Upon death, the spirit of a living thing shatters. Shreds of it stay with the physical remains, becoming the spirit of the corpse (or the spirits of the pieces of it). What happens to the rest varies. In some cases, however, it is known to remain more-or-less intact as a free-floating spirit, or ghost. Other times, the shattering of the spirit happens along less neat lines. Usually, this results in one or more ghost-like entities that have varying degrees of sentience. Rarely (but notably), a piece of the animating spirit stays with the body, resulting in a ghoulish animated corpse.
Through necromantic magic, shreds of animae (usually collected in places where much death has occurred) can be forced to join with the inanimae of corpses, resulting in animated corpses. Alternately, some necromancers can tie their own spirits to the inanimae of corpses, controlling them remotely as gruesome extensions of their own body.
It is possible to use such techniques to bring the dead back to life – finding the pieces of their spirit and carefully putting them back together with those of a healed corpse. This works better when done closely to the time and place of death, as the pieces of the shattered spirit will be easier to retrieve and suffered less deterioration. These rituals are difficult – and dangerous. If some of the spirit-pieces are lost or damaged, the result may involve the resurrection of something that is incomplete and, possibly, not fully alive.