since nobody has offered anything yet, i will provide a brief on what is known about the "Sinister NPC"

First, he is unable to speak except for those occasions where he can. Recently Nailgus discovered that he is being possesed by an entity known only as Voltiz when he does. Voltiz seems to think he is powerful, but does not seem to be very clever.

Second, his robes are of a style that was considered odd, but not too far from a desert aba (read arabian traditional), and he carries a short curved walking stick (which could be a magic staff or rod, though it has no magical aura hte spellcasters of the group can detect).

Third, he has proven capable of casting silent magic misile 3 times in one battle, and once (possibly while possesed) cast an unknown spell (Nailgus is not learned in such matters and was the only witness) which allowed him to deliver a touch that disintegrated their foe.

Fourth, while he has not proven extremely stealthy in most circumstances, he has an odd habit of slipping away un noticed.

Finally, and possibly most disturbing, in the introductory game, he came into town with several other men who apparently lined their boots up in the closet and summarily vanished off the face of the planet. Again, this happened while he was able to speak, and so might have been the same disintegrating effect seen later.

so there you have it... as more is revealed about this person (and the plot in general) i will add details here.