The first layer of the Nine Hells of Baator The most likely beachhead for any attack by demon-kind, it was the primary battleground of the Blood War: legions of devils marched across its plains in continual readiness to repel the hordes of demon invaders that sailed the River Styx into the layer.
The layer was a charred wasteland of vast ashen plains covered in rubble and the occasional line of foothills and basalt mountains heaped with broken rocks of obsidian and quarts. The ubiquitous presence of rocks and boulders, some of which seemed to resemble tormented faces and shapes of creatures, rendered the terrain extremely treacherous and difficult to cross at any pace quicker than a fast walk.
A blood-red light suffused the very air and huge fireballs flew at random through the sky, occasionally impacting and exploding wherever they hit. There was no sun or stars in the Avernus sky. Travelers needed to get to shelter like a building or cave, lest they inevitably be struck.
As with all the lower planes, the River Styx ran through Avernus, with a number of offshoots and falls. Rivulets, lakes, and streams flowed across Avernus's plains and fed the Styx. The source of this blood was not known; devils claimed it probably came from all who had died on Avernus.
The layer's territory was also in a constant state of expansion by military conquest. The Styx, which at one point flowed at the edge of the layer, was later located at its center thanks to a relentless baatezu campaigning and conquering of gate-towns along the layer's edges.