From the introduction:
This fortress was the center of power for a vampire cult dedicated to the fallen goddess called Niosha, who was cast from the heavens for her predilections upon other young gods. Entrapped nearby is the remnant of one of her deity lovers who also fell from grace, Ravadin, set to guard the tomb. Since the arrival of Madrasheen, a powerful devotee of Niosha, the power of Ravadin has diminished even further to barely a whisper.
The upper levels of the prison are suitable for mid-level adventurers (approx. 11th level). The lower levels of the prison require a high-level party.