This accessory provides a wealth of highly detailed information about one of the most popular regions in the Forgotten Realms world, the world beneath the ground. It includes details on the most popular Underdark race, the drow, in addition to multitudes of other below-ground species. In addition to regional feats, prestige classes, spells, monsters, and magic items, there is also background content on this area of the world, including extensive story content gathered from a multitude of Forgotten Realms products and articles. To aid Dungeon Masters, the material for running a campaign is isolated in a single section of the book and includes adventure hooks.