From the back of the book:
High-Level Threats for Your Campaign
Your player characters stand on a precipice, the corpses of slain enemies all around them. Across a great expanse awaits the final threat - their ancient nemesis. What terror will you unleash upon them?
This supplement provides Dungeon Masters with truly wicked threats to challenge high-level adventurers. Each "elder evil" featured in this tome comes with a detailed description, statistic blocks for the elder evil and its minions, tips for how to incorporate the elder evil into any D&D campaign, adventure seeds, and maps of the elder evil's lair (complete with encounters and room descriptions). This book also provides guidelines to help DMs create their own unique elder evils as worthy campaign villains and endgame encounters.