Fiendish Codex I: Hordes of the Abyss is an optional supplemental source book for the 3.5 edition of the Dungeons & Dragons roleplaying game.
Explains a number of well known aspects of demons, including their physiology, their interactions on the Material Plane, the roles they fit into, and how possession works. Also details the Black Scrolls of Ahm.
Discusses the difference between the three main subtypes of demon: loumara, obyrith, and tanar'ri. Also includes information on 16 different types of demons: Armanite, Bar-Lgura, Broodswarm, Bulezau, Chasme, Dybbuk, Ekolid, Goristro, Guecubu, Lilitu, Mane, Molydeus, Nabassu, Rutterkin, Sibriex, and Yochlol.
Discusses the following 14 demon lords: Baphomet, Dagon, Demogorgon, Fraz-Urb'luu, Graz'zt, Juiblex, Kostchtchie, Malcanthet, Obox-ob, Orcus, Pale Night, Pazuzu, Yeenoghu, and Zuggtmoy. Their statistics are given at lower Challenge Ratings than in previous books such as Book of Vile Darkness, ranging from Juiblex at CR 19 to Demogorgon at CR 23. It also provides tips on advancing the demon lords so that they can be as powerful as you like.
Discusses various ways of interacting with demons, from fighting them to allying with them.