Mulbind-Fel is a massive hole diving down from Gunaegail into the subterranean reaches of Raibyr and those beyond. From the leering walls of Ilbaragûn, the hole has a mouth two miles across, narrowing slightly as it plunges for more than eighty miles.
Steep in descent, roughly eighty-five degrees, the upper tracts of this area serves as the heart of the evil dwarven kingdom of Rethmorg. A common belief, a religious inspired one, is that at the very bottom of Mulbind-Fel is a rift to Tarâk's realm. Those that have actually went there say such legends have no merit, "a trash pile, a graveyard, a mound of bones from all the sacrifices tossed in the name of Tarâk."
Mulbind-Fel's upper tracts is a veritable maze of passages, labyrinths, crypts, and mining tunnels. The latter are collectively known as the World Copper Core. There is so much copper coming out of this area, more than the markets can handle, that all the excess is stored across dozens of vaults.
With many holds and peoples living near it, Mulbind-Fell is a dumping ground, with excess water, sewage spillways, sludge from blacksmithing, and trash making their way downward. From the top, these sort of things coming from the looming walls of Ilbaragûn, then joined by the dozens of holds and settlements that dot the hole's descent. A lot of this stuff, "one man's trash is another man's treasure," is collected and reused further down, even the sewage is put to use in fungi gardens.
The quickest method of descent in this area is by way of cranes, elevators, catwalks and bridges. They pass through towers, villages, and holds of first Rethmorg, then those of the Mîmêk and others subterranean peoples.