Underdark

The Underdark is a fictional setting which has appeared in Dungeons & Dragons role-playing campaigns and Dungeons & Dragons-based fiction books, including the Legend of Drizzt series by R. A. Salvatore. It is described as a vast subterranean network of interconnected caverns and tunnels, stretching beneath entire continents and forming an underworld for surface settings.

The Underdark featured prominently in the campaign settings World of Greyhawk and the Forgotten Realms. The concept of a dungeon that spanned a planet was first introduced by Gary Gygax in his D-series of game modules and at the end of the G-series. The Underdark was described in detail in the 1986 manual Dungeoneer's Survival Guide, by Doug Niles. It was also part of the Eberron campaign setting, in which it was called Khyber and was home to evil beings driven deep into the caverns at the end of the age of demons.

A review for Pyramid refers to the Underdark as "one of the most well known facets of the Forgotten Realms". Much of the literary attention for this deep underworld stemmed from the sourcebooks and accessories for the Forgotten Realms setting, including R.A. Salvatore's novels about the fictional character Drizzt Do'Urden.

The Underdark was also the setting for the expansion pack to the computer game Neverwinter Nights called Hordes of the Underdark, which featured the port city of Lith My'athar, and the mysterious Seer.

...
Wikipedia