The heroes explore the lost underground serpentfolk city of Ilmurea only to discover that it is not nearly as abandoned as they had thought! Feral descendants of the original Azlanti inhabitants of the surface City of Seven Spears have degenerated into subterranean morlocks, devolving into madness and evil over thousands of years.

Malign creatures from throughout the Darklands flock to Ilmurea to prey upon these dwellers, and serpentfolk from around the world flock to Ilmurea, beckoned by the severed head of their slain ophidian god, Ydersius.

A grim ritual, already in progress, will use the power of the Serpent's Skull to restore the serpentfolk of Golarion to their true, terrible potential, instantly creating a new menace below all the nations of the world. A lost Pathfinder holds the key to disrupting the ritual, but before his plan can be executed he must first be rescued from his underground prison!