In the final step of this quest, you are given a choice about siding with Bartolomaeus, or refusing. Upon refusal, you slay him and plunge the Ancient Dunmer Blade quest item into him, releasing The Fiend. The Fiend gives you as a reward the blade you just plunged into Bartolomaeus' body, enchanted with his trapped soul.
The problem is that the actual quest reward given is a spear, not a long blade as the Ancient Dunmer Blade had been.
This contradicts 1) the prior quest item, 2) the spoken dialogue of The Fiend and Bartolomaeus, 3) the journal entries for the quest.