Darkness in the Delimbiyr
The Underdark is the vast network of underground caverns and tunnels underneath the surface of Abeir-Toril. It is home to a host of evil beings driven deep into the caverns at the end of the Days of Thunder, but many were also attracted by the strange magical radiations, known collectively by the drow word, faerzress. The portion of the Underdark directly below the North, more specifically, the High Forest and the Delimbiyr Vale is often referred to as the Northdark.
The Underdark is extremely dangerous, especially to those not native to it. There are the usual dangers associated with caverns: claustrophobia, deadfalls, poisonous air or water, and getting lost in seemingly endless tunnels. There is no light except for occasional patches of fluorescent fungus and thusly most native inhabitants either have highly developed senses or adapted darkvision over the ages. Food can be extremely difficult to find, and much of the natural vegetation is poisonous, hostilly sentient or inedible.
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. Survival is incredibly fierce and power struggles and death are commonplace.
Levels of the Underdark
To those well-learned of its geography, the Underdark is actually divided into three distinct layers: