From the back of the book:
A knife flashes in the darkness. You never see the face of your attacker. The adventure begins.
Their spirits trapped by a cursed magical weapon, the heroes find themselves imprisoned on the surreal domain of Aggarath. Here, they must explore the mind of Chardath Spulzeer, the insane lord of the realm. Somewhere in his shattered dreams and tormented memories lie secrets that may enable the adventurers to escape.
The Forgotten Terror is a new adventure for the Ravenloft campaign setting. It can be played as a stand-alone scenario or as a sequel to the events depicted in the Forgotten Realms Adventure Castle Castle Spulzeer. The unique nature of this domain and scenario makes The Forgotten Terror easy for the Dungeon Master to run without forcing players through a linear plot. Alternate endings allow the DM to link this adventure to Ravenloft, the Realms or any other campaign world.