From the back of the book:
Every Man Has His Demons
For years, the knights of Mendev have held back the evil locked inside the Worldwound, a vast, demon-tainted landscape. Every few years, they anoint a new protector, a sacred guardian charged with pitting his will against the teeming hordes bent on devouring the very souls of the living. This year, something has gone horribly wrong, and now a bloodthirsty demonic army has overrun the ancient fortress, and threatens to pour into the surrounding countryside.
The Demon Within is a dungeon-based adventure for 11th-level characters compatible with the world’s most popular fantasy roleplaying game. This adventure includes all the information you need to send your heroes into the belly of a demon-infested keep littered with lone survivors, desperate holdouts, and fallen heroes.
This adventure is set in Mendev, the site of the largest crusade in the Pathfinder Chronicles Campaign Setting, but it can be easily adapted for use in any setting. For more information on this enduring struggle, check out the Pathfinder Chronicles Gazetteer.