When you reach the tower where the princess is held captive, you find a withered woman dressed in silks, gems and a delicately coiffed wig. Lipstick would do little to emphasize something that's long since shrunken back so close to the skull. And what would eyeliner do to eyes already so shadowed within such dark eyesockets? You'd be horrified, but your mission was to rescue the princess of Nocturnus, city of the undead.
Necropolitans were a new player race introduced in the horror-themed Libris Mortis. The idea with them is that you asked already-existing necropolitans for permission to join their numbers. If they accepted you, you go through a long and painful process where life energy is drained through your body and is replaced with negative energy. Unlike, say, a lich, another intelligent undead whose own willing transformation from living to dead involves concentrated evil, necropolitans aren't necessarily members of the dark side.
The book kinda implies that they're just really reserved, serious scholars who'd prefer to be left alone by anyone who thinks the undead are shambling abominations.