From the introduction:
The Book of Fiends provides an in depth look at evil in all its varied forms. From the uncontrollable Harbinger of Vengeance to the perverse Philotanus the Seducer, every conceivable evil has some form of representation. This web enhancement helps to bring this evil into you campaign world, either as a player of an evil character, or as a GM with a flair for surprising your players, by presenting comprehensive summoning tables for all the fiends of the Book of Fiends along with fiends and fiendish animals from the MM. In addition, this web enhancement updates the daemonic, demonic, and devilish templates from the Unholy Warrior’s Handbook to the 3.5 rules, allowing you to create hundreds more nasty surprises for your campaigns. Finally, the epic unholy warrior, in all his brimming evil concludes this expansion. Enjoy.