Shadows of New Jerusalem
The undead are the denizens of the Underworld and the citizens of Necropolis. They are the unquiet dead, the small proportion of mortal souls who found they gained a second life after their first ended.
The undead are unique in the Other Realms because they were all once human; they are the few souls that found a second, half-real life after their time on Earth had ended. (Almost all mortal souls simply vanish into the ether upon death; undeathliness is a rare event.) But the undead only get one extra shot at existence; if they die a second time, it’s forever. So they depend on humanity for repopulation, and seek a way to swell their numbers before they dwindle away.
The undead miss their mortal lives, but they can normally control that yearning. If afflicted by human attention, they may be overwhelmed by hunger for that life; a mild-mannered revenant becomes a zombie hungry for brains, or a vampire starving for blood. As denizens of a twilight, dying world, they are also burned by sunlight – another reason to avoid the light of day.