Avernus.
The first layer of Baator Hell was a scorched, rubble-strewn wasteland, its monotonous landscape broken by mountains and hills with sloping sides. Countless armored devil legions stood eternal guard here, forever mustering for extraplanar assaults in the Blood War. Blood-red light filled the air, and fireballs sometimes streaked across the sky before crashing down in terrifying explosions. Rivers filled with blood flowed through Avernus, eventually joining the main course of the River Styx. No one knew where this blood came from, though the devils claimed it might belong to all of Avernus's past victims.
The central structure of this layer was the Bronze Citadel, a rough, sturdy, rigidly laid-out city covering more than thirty square miles, renowned for its twelve concentric rings of walls, each lined with war machines. This devil city teemed with planar incarnates, both lemures and soul shells, as well as tens of thousands of lesser devils of every kind, ready to march into the Blood War at any moment. In the past, Avernus had been the first beachhead to bear the brunt of massive demon offensives, and its fortifications were constantly reinforced within the Bronze Citadel. Toiling soul shells, lemures, and imps continually expanded the city.
Near the Bronze Citadel stood the [Pillar of Skulls], an enormous landmark made of skulls that symbolized the evil spoils won in the Blood War.
Most of these trophies were twisted demon skulls, ranging from tiny specimens to ones the size of houses. This terrifying pillar rose more than a mile high and contained the heads of countless powerful demons, all trophies claimed by devils in the Blood War.
The lord of Baator Hell's first layer was Dispater.
A legendary powerhouse who had risen from the ranks of the weakest devils, Dispater was a powerful pit fiend who ruled