Death is not always the end. Through various machinations of fate or intent, a creature can remain in the world after its death as a plague on the living, or something worse.
Sentient living creatures have a body and a soul, the latter of which is the consciousness that exists in and departs from the body when it perishes. The positive life force that fills a creature’s living body, driving its muscles and emotions, is called the animus. The animus provides vitality and mobility for a creature, and like the soul, it fades from the body after death.
Through a number of different ways, a negative animus can rouse the body after death (thus preventing the soul from knowing peace) and in some cases, can even exist apart from the body as a cruel memory of life. Such impetus can come from necromantic magic, a corrupting supernatural influence at the place of death or internment, a locale’s connection to the Shadowfell, or extreme rage or fear at the time of death.