I remember reading a fantasy horror, and I lost the book a while ago. It was about some sort of monster that drove the inhabitants crazy and unleashed nightmarish creatures within a keep.
Plot points I remember:
A group of adventurers are walking along a forest road, and come across a keep or fort. The place is eerily abandoned.
They come across a large room, and it is a gory mess, apparently the remains of a lot of the inhabitants of the keep.
The spell caster of the group is a wild-like, female sorcerer. It seems her magic was limited by the fact that it exhausted her, and she was pushing herself to her limits towards the end of the story.
The leader, a sort of knight style fighter, finds the monster somewhere in the basement or dungeons. It is described as hideous and kind of a nightmare itself.