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Faerun's Underdark includes drow cavern-cities, sunken aboleth strongholds, entire realms of mind flayers, and wondrous vistas shrouded in everlasting darkness. This section describes some of the most notorious, important, and incredible domains in the Realms Below.
The domains described here do not represent an exhaustive list of the Underdark realms. Uncounted hundreds- perhaps even thousands â€" of deeply buried caverns and lightless cities exist beneath Faerun, and virtually every surface-world dungeon seems to connect to murky depths from which all sorts of horrors can spew forth. Consider the places discusses in this section to be ones that knowledgeable surface folk know of, and a sampling of the types of places that deep-delving heroes might explore.
Note: As described in this section, the Underdark is a vast, largely unexplored place. This leaves players with a multitude of different ways to explain how their character found their way to the town of Sanctuary. With no way of telling exactly where the setting lies beneath the surface world, as nobody who has made their way to the surface has returned, characters could hail from almost anywhere in Toril. For example, someone taken as a slave during a surface raid in Waterdeep could very well escape and find a tunnel that leads from the Northdark to the Old Shanatar.
Players are encouraged to develop concepts that they find interesting and not worry about exactly where the setting lies within the Underdark in relation to their character's surface home.
Underdark Domains
Faerun's Underdark is not a single, continuous cavern system, and even places on the same level may not connect to each other. The existence of Underdark settlements beneath Waterdeep and Raven's Bluff, for example, does not mean that it is possible to travel from one city to another underground. In fact, the Underdark consists of a number of discrete domains. A domain is simply a collection of Underdark locales among which underground travel is reasonably easy. In certain places it may be possible to trailblaze a route from one domain to another, but then again, it may not.
An Underdark domain is something like a large island in the surface world. It may feature several distinct terrains and cultures, but it's possible to travel all over an island without having to set sail on the ocean. Similarly, two locales in a single domain are connected by enough cave systems, tunnels, and other passages to enable travel from one to the other without leaving the Underdark. Cities and settlements in the same domain are much more likely to engage in diplomacy, trade, or warfare than cities in separate domains. Like the continents or regions of the surface world, domains tend to group disparate cultures together and force them to interact with each other for good or ill.
Faerun's Underdark consists of seven major domains, a dozen or so minor ones, and hundreds of otherwise isolated locales that don't appear to connect (at least not easily) to any other Underdark networks. The major domains include the following.
The Buried Realms
Beneath the mighty desert of Anauroch lie the Buried Realms, the demesnes of the unspeakable phaerimm. Because the Buried Realms are situated below the ruins of ancient Netheril and its daughter states, they contain many Netherese ruins. The presence of the phaerimm, however, prevents most fortune-seekers from venturing into this portion of the Underdark, and very little is known of any particular sites of cities here.
The Darklands
In the center of Faerun, beneath the Dragon Coast and portions of the Orsraun Mountains and Turmish, lie the Darklands. This immense domain stretches from Cairheim (the realm of the Dodkong) in the west to the foul warrens to Traaskl Thorog beneath Chessenta in the east.
The Deep Wastes
East of the Buried Realms lies the small domain known as the Deep Wastes, which runs beneath the Dalelands and the Moonsea. Sparsely populated and largely wild, the Deep Wastets are home to the now-ruined drow city of Maerimydra, which once held Shadowdale as a conquered surface land. The Deep Wastes extend as far as the Eathspur Mountains beneath the Vast. Few easy access point exist, however, so large portions of the domain are poorly explored.
The Earthroot
Far to the east, below the plateau of Thay and the towering rampart of the Sunrise Mountains, lies the vast domain know as the Earthroot. Very little is known about this domain, and since it lies beneath the Unapproachable East, few western folk ever even learn of its existence.
The Glimmersea
Below the Sea of Fallen Stars lies a vast Underdark ocean called the Glimmersea, or sometimes the Sea of Starry Night. The Glimmersea lies 20 miles below the floor of the Sea of Fallen Stars. Its horizontal area overlaps that of the surface ocean for most of its extent, although the Glimmersea extends under some of the lands adjoining the Sea of Fallen Stars, such as Altumbel and Aglarond. Similarly, certain portions of the Sea of Fallen Stars, such as the Easting Reach, have no arm of the Glimmersea beneath them.
Great Bhaeryden
In the south-central portion of Faerun, below the endless plains of the Sharr, lies the homeland of not one but two Underdark races â€" Bhaeryden, the great caverns of the south. The first great homeland of the dwarves, Bhaeryden was conquered and occupied long ago by the drow when they first descended into the Underdark. The drow named their new realm Telantiwar, but their kingdom did not last. For some reason that has been forgotten in the intervening centuries, the great vaults that made up the heart of Telantiwar collapsed, crushing a dozen drow cities and creating the Great Rift in the process. Although the cavern that housed Bhaeryden no longer exists, it was only part of a sprawling network of caves and deep delvings.
The Northdark
North and east of Waterdeep lies a large and notorious domain called the Northdark. Many of the Underdark's most famous realms lie in this area. The Northdark is also home to the wreckage of numerous ancient realms. The Buried Realms lie quite near it to the east â€" in fact, the Northdark meets the Buried Realms beneath the ruins of Ascore. Countless orc-holds, troll-caves, and giant-dwellings also lie beneath the Pine of the World to the North.
Old Shanatar
Extending for hundreds of miles beneath Amn, Tethyr, and Calimshan, Old Shanatar was once home to the powerful dwarven realm of Deep Shanatar. Only one of Deep Shanatar's eight kingdoms still stands today; the others have fallen to drow, mind falyers, and duergar. Of the major domains, Old Shanatar contains the most artificial delving, halls, and passages, including works of tremendous scale.