The Underdark is a vast network of underground caverns and tunnels beneath the surface of Corwyn. It is home to entire cities filled with exotic beings and strange deadly monsters roaming the passages between.
The Underdark is actually made up of several distinct regions:
—The Upperdark; which consists of caverns located in the first three miles below the surface. It was here where the surface dwellers and those in the Underdark most often met.
—The Middledark; which consists of caverns located between three and ten miles below the surface, this layer was where most of the Underdark cities were located.
—The Lowerdark; which consists of caverns located deeper than ten miles below the surface, the Lowerdark was where even those who knew the Underdark were loath to go.
The Underdark is not one giant cavern under Corwyn, but rather, many huge networks of caverns and caves. As a result, it is not always possible to travel from one end of the Underdark to the other. The Underdark is divided into several regions that were similar to continents of the surface world above. While it is possible to travel from one place to another within a domain, separate domains tend to have very few passages linking them.
Light sources are almost universally absent in The Underdark, save for the occasional luminous crystals called "Lazurite", or fungi called "Luciferus". Large swaths of the underground world are also completely bereft of food or water, making it a necessity for explorers to carry supplies with them.
The Deep-Gates
One method of travel in the subterranean world below Corwyn is via ancient magical portals called "Deep-Gates." It is a unknown who built the first one, but the Dark-elves copied them and later constructed a network of these devices to suit their own purposes.
Ecology
The Underdark is home to many species, but the most powerful races are: the Aboleth, the Beholders, the Dark-Elves, the Duergar, the Ko-Toa, the Mind-Flayers, and the Svirneblin.
Among these advances races, there exists many large cities, spread across the Underdark of Corwyn:
Drow Elves: 24 city-states
- Aevon-Nhor; House Su'vharen, House Armigo
- Ched Nassad; House Tyralandi, House Daragon
- Erandolin; House Ilathyr, House Vor-Shaval
- Iladris; House Dorin'Viir, House Kel'Arryn
- Karzai; House Sha'haris, House Kas'viir
- Menzoberranzen; House Bäenre, House Xor'Larrin, House Nasadra
- Myrdra; House Vel'Thura, House Sharad'Zhi
- Ord; House Azu'viir, House Ra'Lindi
- Orylon; House Volari, House Kelor'Vaan
- Phaerice; House Kron'vin, House Na'Zhani
- Rel-Mandor; House S'zordrinn, House Ekron
- Sha'Harizad; House Kenarfin, House Abraxi
- Sha-Taar; House Iskarren, House Ra'Shav
- Vhan-Dhorian; House Devario, House Shar'Voon
- Vherak-Nashad; House Jannalor, House Draven
- Za-Hinda; House Quel'lyn, House Xha'rann
- Zirak-Anor; House Azinräe, House Vonnarc
Duergar: 15 cities
- Grunthorigard
- Arskraag
- Buruknir
- Norgrim
- Rhakka
- Traask
- Undrak' Thar
Svirneblin: 9 cities
- Reth Malag
- Kheldarak
- Sefuldra
- Reshov
- Na-Drak
Illithids: 7 cities
- Arvhon-Skyr
- Sharularen
- Ialystyr
- Ashimvor
- Nar’Shoon
- Ekan- Lazhur
- Azathyr
Ko-toa: 12 cities
- City of the Glass Pool
- City of Glimmering Shard
- City of Reflecting Pool
- City of the Purple Chasm
- City of Murky Depths
Aboleth: 5 cities
- Sarnath
- Raxithan
- Ar-Kumool
- Great Shaboath
- Imred-Nharay
