From the back of the book:
Raw Chaos and Pure Evil
Out of the maelstrom of chaos the demons came—a primordial horde of perverse souls consumed by hatred. They are as ancient and infinite as the multiverse itself. Even the bottomless Abyss could not contain their malice, and so they spread out across the planes, corrupting and destroying everything in their path. No living soul is beyond their reach, and with each conquered soul their numbers grow. What can stand against such a terrifying onslaught?
This supplement for the D&D game presents the definitive treatise on demons and their unspeakable home plane. Along with information about the physiology, psychology, society, and schemes of demonkind, you’ll find feats, spells, items, and tactics commonly employed by demons and those who oppose them. This book also provides detailed information on various demons, demon lords, and Abyssal layers.
To use this supplement, a Dungeon Master also needs the Player's Handbook, Dungeon Master's Guide, and Monster Manual.
Expanded by;
Fiendish Aspects I
Fiendish Aspects II
Fiendish Codex I: The Lost Annals