From the publisher's website:
Learn the hideous secrets and vile rituals of Golarion's most evil cults, from the lord of all devils Asmodeus to the apocalyptic monstrousness of Rovagug. These deities are the villains of the cosmos, sadists and manipulators who view mortal life as a resource at best, or cattle to be fed upon at worst. Yet even the gods of corruption do not directly interfere in the workings of humanity—they leave that to their faithful, a despicable lot who spend their lives in the pursuit of cruelty, trickery, and violence.
Faiths of Corruption presents a player-friendly overview of the evil-aligned religions and faiths of the Pathfinder campaign setting, along with new rules and information to help players customize pious characters in both flavor and mechanics.
In this book, you'll find:
Information on each of the major evil gods and his or her corresponding religion, including what’s expected of adventurers of various classes, ways for the faithful to identify each other, taboos, devotions and ceremonies, church hierarchies, unholy texts, religious holidays, and more
An overview of various evil philosophies, including the diabolic Chelish government, the Whispering Way, the shadowcasters of Nidal, the witches of Irrisen, the necromancers of Geb, and the Red Mantis assassins
New feats for evil cultists
Antipaladin codes for several deities
New magic spells inspired by the major evil deities' teachings
Rules for sacrifices to the gods of evil on their sinister sacred days
Details on minor evil deities, powerful fiendish demigods, and more!
Each bimonthly 32-page Pathfinder Companion contains several player-focused articles exploring the volume’s theme as well as short articles with innovative new rules for social, magic, religious, and combat-focused characters, as well as traits to better anchor the player to the campaign.