Har'oloth in Undercommon
The Underdark, called Har'oloth by its denizens and often also called the Worldunder or World Under, is a large realm located several dozen miles under the surface of El-Bayir (though some "mountainous" "highland" regions of the Underdark are much closer to the surface than the "lowlands").
The Underdark was created by the God Donere during the First Age, and is the origin point for many of the monsters in the world. Donere does not directly rule the Underdark, but the large monster presence makes it a domain that largely bends to his will. However, there are several civilized peoples in the Underdark, most notably the Drow, who make up the most organized and populated civilized population in the region.
The Underdark is a notoriously inhospitable realm. It is dotted with near-constantly erupting volcanoes, creating a constant blanket and snow of ash and foul fumes. The lack of sunlight makes seeing and producing food difficult, and the diet largely consists of mushrooms and livestock. The large monster presence makes most places outside of the walls of the cities and towns incredibly unsafe, and this has led to populations being highly concentrated into walled cities and towns. As there are very few exits from the Underdark, the population has been forced to adapt to life there, creating hardy people.
The lack of ore and abundance of monstrosities makes metal armor exceedingly rare. Most denizens of the Underdark wear armor fashioned from chitin and shells of the insectoid monsters they kill. The constant dust and grime also leads the Drow of the Underdark to typically cover most of their bodies to avoid exposure.
