Underdark provides a wealth of highly detailed information about one of the most popular regions in the Forgotten Realms world, the world beneath the ground.

Underdark includes details on the most popular Underdark race, the drow, plus 16 other below-ground character races. In addition to 25 new regional feats as well as new prestige classes, spells, monsters, and magic items, there is also background content on 60 cities and sites of interest, including extensive story content gathered from a multitude of Forgotten Realms products and articles.

To help both players and dungeon masters use the book without players stumbling onto things they shouldn't, additional material for running a campaign is isolated in a single section of the book and includes adventure hooks.