A scary/uncommon undead? Maybe one situation just like the 'two litch in a castle' that there was in NWN1 .... maybe combined with the 'blind god' in BG2.
I try to elaborate a bit:
some 'people' worship a new god. No one has seen this god, statues in the temple has many many forms (some scary). Two priests claim to have seen the god. A couple of things happens, 'trivia quiz' between the heroes and these two priest, time to decide which priest to follow and..... here you are, the two priest (they didn't have memory of this) were undead, both of them were under a disguise spell and the god was a shadow, supported by a necromancer (the spell.....)
A similar situation happened at the group that I mastered..... obviously their characters are all undead now.... guarding the temple (evil laugh). Oh yeah, they had high-level chars, but when you choose poorly, something wrong can happen....
By the way.... a good old undead? Let's talk about the Gods of Lankhmar....