Seven Kingdoms: Seowyn's Crossing
When creatures die, most of their spirits pass naturally into the Shadowfell and beyond. In some cases, however, their corpses are brought back to an unnatural state called undeath, in which they are neither fully living nor entirely dead. From then on, they are essentially immortal and must be laid to rest by more destructive means.
Some undead are brought back by necromancers. These undead tend to be littlem ore than mindless skeletons or corpses animated by the magic and with little to no free will of their own.
Other sorts of undead are created when they die at the hands of another undead. These sorts usually have a tortured fragment of their old self still lingering, even memories and personality.
The vilest sort of undead are those who deliberately seek this state to preserve their lives beyond natural limits. These creatures are almost always evil to the core with no regard for the lives of others.
Undead tend to be very vulnerable to the powers of light. Those with a connection to the Gods are particularly potent in their destruction of the undead.