I debated about whether to include the Ash Vampire in this bestiary or not. Strictly speaking, it is a creature. However, there are only seven in the game and they function much like hostile NPCs. Unless you go search them out, the odds are very long that you'll encounter more than the three you must get past on the Main Quest.
These guys are the top rank of the Sixth House, sitting only slightly below Dagoth Ur. They are truly nasty creatures (as in tough cookies). Their individual statistics vary slightly, so I will present them here only as ranges where appropriate and leave the specifics to the places where you'll actually encounter them.