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 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.

Fiendish Codex I was written by Ed Stark, James Jacobs, and Erik Mona, and was published in June 2006.