An excellent book for tieflings. In there are introduced Legacies, which represents affiliation with a specific devil. There are a dozen of legacies (all are common sins, like lust, gluttony, etc), with a full description, feats and paragon path. It reminds me the good old Book of Vile Darkness. All the crunch is really good and innovative (like a gluttony power which allows to eat material like stone or metal, and gain benefit from it), and seems well balanced. The fluff is really good too.
It is a must have for tiefling character, as well as for any player which wants an "evil" feeling.
[5 of 5 Stars!]