After finding the route to the lost city of Neruzavin, the adventurers must mount an expedition into the vast and deadly desert region called the Parchlands, where they hope to catch up to their treacherous former employer. They brave the deadly desert to arrive in an abandoned city at the edge of a lake where a Great Old One slumbers. The adventurers must track Count Lowls's party, recover a vile tome, activate the city's Star Stelae, and perform a powerful ritual that allows them to follow the Count to Carcosa. Along the way the characters must be careful they don't wake the slumbering Great Old One or stir up the flying polyps sealed beneath the city.

"What Grows Within" is a Pathfinder Roleplaying Game adventure for 13th-level characters. The adventure continues the Strange Aeons Adventure Path, a twisted delve into madness that pits the heroes against the cosmic horrors of the Cthulhu Mythos. A selection of new monsters, details on the great old one Xhamen-Dor, a look at the Necronomicon, and the next installment of a new Pathfinder's Journal round out this volume of the Pathfinder Adventure Path.

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.