Avernus

 is the first layer of the Nine Hells, and the arrival point for most visitors to the Hells.

Description
Avernus is a desolate, rocky layer, full of empty battlefields and rivers of blood. It is cratered from the impact of occasional comets, and the air swarms with biting insects.

Denizens

 * Zariel: a fallen angel and the Archduchess who rules over Avernus.
 * Tiamat: a prisoner on this layer.

The adventures of Arkhan the Cruel after the banishment of Vecna haven't been established as Critical Role canon, but according to Joe Manganiello, Arkhan has a keep on Avernus near his deity's prison. After the defeat of Vecna 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.