With an army of their own, the heroes finally wage war against the Ironfang Legion in the climactic conclusion of the Ironfang Invasion Adventure Path! To end the hobgoblin threat once and for all, they must assault the Onyx Citadel—an ancient fortress on the Plane of Earth constructed eons ago by the mysterious Vault Builders—explore its alien landscape, and undermine the inhuman forces. Can the heroes fight their way past the Ironfang Legion's greatest leaders and confront General Azaersi before she activates her ultimate weapon: A doomsday device that will wipe out not only Nirmathas, but the surrounding nations as well?

"Vault of the Onyx Citadel" is a Pathfinder Roleplaying Game adventure for 16th-level characters. Ideas for further adventures expanding on the consequences of the Ironfang Invasion campaign, an examination of the secrets and legacies of the mythic Vault Builders, and a selection of new monsters 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.