Bastion of Broken Souls

From the back of the book:

"The Blood of Ancient Battle Rises

An eternal demonic war draws dangerously close to an end, threatening the balance of all existence. At the crux of this chaotic feud resides a legendary terror fixated upon immortality at any cost. Only the most powerful heroes can hope to defeat a force that lies beyond the influence of the gods.

Bastion of Broken Souls is a stand-alone adventure for the DUNGEONS & DRAGONS game. Designed to challenge 18th-level D&D heroes, it pits them against some of the most powerful beings in the multiverse."