From the PDF introduction
Sample fiends for use with Fiend Folio
The new Fiend Folio book has an emphasis on, logically enough, fiends: outsiders with the evil subtype. So great is this emphasis that the book even offers new ways to enhance the powers and abilities of such creatures, particularly as they pertain to their interactions with mortals.
The Fiend Folio offers three new prestige classes designed especially for fiends-the fiend of blasphemy, the fiend of corruption, and the fiend of possession. This article explores the application of those prestige classes by offering detailed statistics and brief histories for several fiends that have adopted them. Each prestige class is illustrated with two fiends, one from the Monster Manual and one from the Fiend Folio. These creatures are ready-to-use NPCs that you can drop into your campaign world wherever you need a real challenge for your players!