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.