Throughout history, the teeming hordes of the Abyss and the strictly regimented legions of the Nine Hells have battled for supremacy in the cosmos. In the mortal world, the scant few scholars, arcanists, and adventurers who know the conflict for what it is refer to it as the Blood War.
The fighting takes place across the Lower Planes, on the Material Plane, and anywhere else that demons and devils might congregate. From time to time, demons spill out of the Abyss to invade Avernus, the uppermost layer of the Nine Hells. While the devils defend their home turf, they also make strikes against locations in the Abyss. Although the intensity of the conflict waxes and wanes, and the front lines of the war can shift drastically, a moment never goes by when demons and devils aren’t battling each other somewhere in the multiverse.