From the PDF introduction
A Book of Vile Darkness Web Enhancement
The Book of Vile Darkness details all aspects of evil as they might apply to your campaign. This includes, but is not limited to, the statistics and descriptions of those most evil beings, the archfiends-the Lords of Hell and Demon Princes. They each come with their own details plus descriptions of their followers and cultists. Yes, although the archfiends are not true deities, cults of mortals revere them all the same. (The Book of Vile Darkness describes how this works and suggests relevant variant rules.)
Presented here for you are three more archfiends that didn’t make it into the book: a deposed archdevil and two demon princes-well, one demon prince and one princess.