Finding a legendary tomb requires tracking an equally legendary cave worm said to dwell there. As the adventurers set out from Highhelm, they must brave the Darklands' dangers, navigate a treacherous fey city, and recover the relic that retraces the cave worm's path. Yet in doing so, they learn of nefarious rivals who also seek the tomb--rivals the adventurers might stop by infiltrating an unforgiving underground metropolis. "Cult of the Cave Worm" is a Pathfinder adventure for four 5th-level characters, continuing the Sky King's Tomb Adventure Path, a three-part monthly campaign in which a group of adventurers travel deep underground to discover a legendary dwarven king's tomb, seek subterranean treasures, and heal millennia-old injustices--all while stopping an ambitious villain from weaponizing those same discoveries. This adventure also includes a study of cave worm ecology, a gazetteer of the duergar city Hagegraf, potent relics, and several monsters to threaten underground explorers. Each monthly full-color softcover Pathfinder Adventure Path volume contains an in-depth adventure scenario, stats for several new monsters, and support articles meant to give Game Masters additional material to expand their campaign. Pathfinder Adventure Path volumes use the Open Game License and work with both the Pathfinder RPG and the world's oldest fantasy RPG.