Fiendish Codex I: Hordes of the Abyss is a comprehensive sourcebook covering the most dangerous (and popular) fiends in the Dungeons & Dragons cosmology: demons. It provides detailed information about the powers, tactics, and organization of the forces of the Abyss -- both those covered in the Monster Manual and entirely new foes. In addition, this book provides new rules, feats, tactics, spells, and equipment for characters who battle fiends. Extensive story and campaign elements add dimension to playing or fighting creatures of this type. Included is extensive information on the layers of the Abyss and the ruling demon lords, demonic possession, and how to use demons in a campaign.