The Underdark or Kazalhan in the original Dwarven (lit. Darkness Beneath) is a massive, interconnected series of caverns and warrens spanning the entire underground of Eregia. It is said that there are a thousand entrances to the Underdark, scattered all over Eregia, in caves, mountainside and mines, creating one enormous, interconnected system of tunnels and caverns.
The Kazalhan is the ancestral homeland of the dwarven people and once, in prehistoric times, they dwelt in a large cavern, a paradise they call Jëbebror, in utopian peace and security. In time, they began to branch out, discovering precious metals and carving themselves into their many Houses until, in the year 100, a massive earthquake tore open a might fissure near the center of the dwarves’ subterranean realm. Called simply the Chasm (Kazam in Dwarven), the seemingly bottomless pit began to spew forth all manner of horrific, tentacled aberrations, in such numbers as to eventually force the dwarves to abandon the Underdark and colonize Eregia’s surface.
To this day, the majority of the Underdark continues to be plagued by these aberrations, though the dwarves have, very slowly and very gradually, begun clearing and reclaiming sections of their ancient wealth and power, hoping to reclaim their homeland someday.
The Nameless Company once ventured into the Underdark, on the trail of the missing Culas of House Topaz and the great cavern of Jëbebror. Departing from Myto, they traversed many leagues of tunnels, fought aberrations, derro, House Onyx dwarves and purple worms, visited the cities of Adok Zyzed, Adok Dycûn and even crossed the Ozepolhan. In the end, they were reunited with Culas of House Topaz and discovered Jëbebror. (Interlude – Episode 106)