From the back of the book:
In League With Hell
Seventy years ago, the Thrice-Damned House of Thrune defeated its rivals in a bloody civil war following the death of the god of humanity, bringing order to the war-torn nation of Cheliax—but at a price. Now the imperial bureaucracy enforces devil-worship as the state religion, and the common people bow their heads in perpetual servitude to the dark lord of Hell. Fortunately, Cheliax still has heroes: crusaders and revolutionaries motivated by glory, honor, greed for the throne, or hatred of what their land has become under the infernal thumb of Asmodeus.
This Pathfinder Companion describes the nation of Cheliax, its diabolical capital of Egorian, and its shadow-haunted former capital of Westcrown. Inside you’ll also find new character traits, magic items, spells, combat feats, and Chelish NPCs suitable for cohorts or contacts. This book is perfect for use in the Council of Thieves Adventure Path, set in the city of Westcrown, or any adventure taking place in a nation of devils and decadence.