|“|| 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.
- "Organizations of the Underdark", a free web enhancement by Gwendolyn F.M. Kestrel
- Dungeon Masters Guild product page