Dead beings who persist on moving about or generally making their presence knwon in the world of the living.
The most common undead are the animated skeletons who are considered somewhat unnceccessary by most drow since slaves can do just about the same job, only better since slaves are smarter and can be given more complex instructions. Nevertheless some necromantically inclinded Mages create skeletons for menial tasks, tireless guards and suchlike.