Nine Hells

The Nine Hells is the home plane of the devils. It is a plane of evil and unlike the Abyss is highly organized with a strict caste system.

Avernus
Avernus is the first layer of the Nine Hells, and the arrival point for most visitors to the Hells. Avernus is a rocky wasteland with rivers of blood and clouds of biting flies. The archdevil who rules over Avernus is the fallen angel Zariel. Her advisor, a pit fiend named Bel, was replaced by Zariel after Bel fell out of favor with the Lord of the Hells. Zariel's soaring keep is the Basalt Citadel, and her once-beautiful skin and wings have been ruined by the fires of her own evil alignment.

The Scaled Tyrant is a prisoner on this layer of the Nine Hells, confined to her domain by an ancient contract with the Lord of the Hells. Arkhan the Cruel has a keep on Avernus near his deity's prison. After the defeat of the Whispered One in Vasselheim, Arkhan took his newly-acquired Hand of Vecna and Teleported back to his keep in Avernus. As of Campaign 2, he is still trying to unlock the Hand's power in order to free the Scaled Tyrant and unleash the Dragon Queen upon the other planes of existence.

Iron City of Dis
Dis is the second layer of the Nine Hells. The archdevil who rules over this layer is Dispater, who styles himself with the title of "Iron Duke". At the center of the city is his home: the Iron Tower. The Iron Tower is an impregnable fortress, and the paranoid Dispater has enchanted it so that it is equidistant (the tower is always far off, no matter how close to the city center an individual might try to get). Dispater never leaves the Iron Tower unless directly commanded by Asmodeus. When he does so, he always has at least nine different backup plans and escape routes prepared.

Trivia

 * Avernus has yet to appear in either campaign of Critical Role, beyond Joe Manganiello's information about Arkhan. This article assumes that the canon Dungeons & Dragons description of Avernus is the same as the one in Critical Role's multiverse.
 * The Blood War between devils and demons has been confirmed as canon in the Nine Hells of Critical Role's multiverse, both by an untrustworthy glabrezu named Jurazel and by the tapestries found on the walls of the Circlebound Chamber within the Heirloom Sphere.