The Tome of Terrors

From the website: "The Tome of Terrors™ is an encyclopedia of monsters taken exclusively from mythology and folklore. It also contains detailed descriptions of all the various sub-races of the standard LQ races. For example, did you know that, according to folklore, there are a number of elven families, each having its own unique personality? You'll find the light-hearted natures of the Tuatha De Dannan and Liosalfar elves quite distinct from those of the dark and brooding Svartalfar or the murderous Sluagh Sidh. Dreaming up gripping adventures for your players proves far easier when you have a solid foundation to build upon, and the Tome is as solid as it gets. The Tome of Terrors is needed only by the game referee and, in fact, should be avoided by all other players to ensure the greatest enjoyment of the game."