The Underdark is the vast network of underground caverns and tunnels underneath the surface of the world. It is home to a host of evil beings driven deep into the caverns at the end of the age of demons.

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 (insane humanoids)
• drow (dark elves)
• duergar (gray dwarves)
• dwarves
• illithids (mind flayers)
• kuo-toa
• svirfneblin (deep gnomes)