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Forgotten Realms Wiki has an article about: Underdark

The Underdark is a subterranean underworld beneath Abeir-Toril, a strange alien landscape populated by bizarre creatures and ancient races, nearly all of whom are hostile to most surface-dwelling life.

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 beholders, derro, drow, duergar, dwarves, illithids, kuo-toa, and svirfneblin.