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.