Hm. This may be slightly spoilerish (???)
If i'm not mistaken, there are cultists of some kind (or were they skeletons?) standing/kneeling/hanging around in there somewhere. They perform some sort of ritual. You have to dispose of them first to make mr.D vulnerable to any kind of attack. This kind of 'fighting complication' is quite common in NWN.
I hope this is the answer to your problem
