We all like villainy, how can there be hereos without it? The best villains need a compelling motive otherwise it's all just twitches and moustache twirling. One of the standard motives is the villain trying to bring some prophecy to pass. It's a good one cause it excuses all sorts of craziness. Why is the villain killing red headed kids? Ancient Prophecy. Why does he want the sword of omens? Ancient Prophecy. The great thing is that it doesn't matter if the prophecy is legit or not as long as the villain believes in it.
The Book of Bright Darkness is a good role playing tool to help motivate people into dastardly deed. It looks like a large leather bound book that has been scorched in several places. Inside it reads like a demented dream diary by someone of truly questionable sanity. More importantly there are 23 prophecies spread through out the book. I won't say what the prophecies are since any game master can fill it with whatever he or she wants. As a bit of advice don't reveal all the prophecies at once, it allows the book to be used more than once and in different ways by different people.
The trick with the book is that it inspires belief in itself. Think of it as a form of mind control with one command "Believe in me." Anyone that hears or reads it will fall pray to it unless they have strong will power. Anyone actually holding the book will almost assuredly fall into the clutches of books prophecies. Does the power of the book mean that the prophecies are real? Or is it just that the delusions of the author were so powerful that they now reach out through the pages of the book? That is for the game master to decide.