The Underdark is a geographical feature common to most of the published fictional settings of the Dungeons & Dragonsrole-playing game. It is a vast subterranean network of interconnected caverns and tunnels stretching beneath entire continents, perhaps even whole planets. A literal underworld, the Underdark is a strange alien landscape populated by bizarre creatures and ancient races, nearly all of whom are hostile to surface-dwelling life. The Underdark is featured prominently in the world of Greyhawk (where it was first introduced) and the Forgotten Realms. It is also a part of D&D's newest setting Eberron, in which it is called Khyber and is home to a host of evil beings driven deep into the caverns at the end of the age of demons. However, the majority of gaming material describing the Underdark (in the form of sourcebooks and accessories) has been published for the Forgotten Realms setting.
The Underdark is extremely dangerous, especially to those not native to it. There are the usual dangers associated with caverns: claustrophobia, occasional poor air circulation, and getting lost. There is no light except for occasional patches of fluorescent fungus; most Underdark inhabitants either have highly developed senses other than sight or have developed infravision. Food can be extremely difficult to find, and much of the natural vegetation is poisonous.
In addition to those dangers, the Underdark also is the home of many predators and often-hostile races, including:
