Not to be confused with the 1st edition AD&D Fiend Folio.
From the back cover:
Vanquish Terrible Evil
The noblest heroes must contend with the foulest monsters. Whether torn from the darkest planes of existence or spawned as blights upon the natural world, the creatures bound within these covers will challenge stalwart adventurers of every experience level.
This accessory for the D&D game captures over 150 monsters, including some of the most diabolical beings imaginable. While focused on extraplanar and otherworldly creatures, you'll also stumble across new creatures of every type, with Challenge Ratings that range from 1/8 to 25. Along with three new fiendish prestige classes, six new templates, and rules for swarms, grafts, and symbionts, the Fiend Folio offers a multitude of challenges for every hero.