Avernus
First layer of hell, ruled by Zariel. Once it was a bustling realm filled with cities, trade outposts, and other features, but recent activity in the Blood War has reduced it to a blasted wasteland. The few structures still standing are citadels constructed by the devils to repel attackers, to be rebuilt each time the front line of the war moves. The devils are in control of Avernus at present, though the fighting goes on (as it always does) in isolated locales throughout the layer.
Adventurers able to avoid the other occupants can find treasure within some of the ruins, though they must venture to the fringes of the active conflict to have any hope of finding places that have not yet been looted. Many of the active citadels on Avernus loom over the River Styx or surround portals to other planes or to Dis. An amnizu presides over each location, directing the lemures that emerge from the Styx, marshaling forces against invaders, and preventing unauthorized travel.