The Blood War - Mordenkainen's Tome of Foes

The Blood War. 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. The Great Dance We measure ourselves by those we fight, and my company faces the greatest threat to the cosmos. The battle lines in the Blood War undulate like a writhing snake. Each time one side gains an advantage, invariably its lines weaken somewhere else. Demons enter the Nine Hells by following the River Styx from the Abyss into Avernus. By hiring skilled merrenoloth pilots to navigate the river, a demon lord can transport an invasion force of enormous size. Mimicking a complex dance, the two sides shift their attacks and trade positions with each passing day. About once every thousand years, the demons close in on the lower reaches of the Styx and portals leading deeper into the Hells. Invariably, the legions of the Nine Hells boil up from below to repel the invaders out of Avernus, back to the juncture where the Styx enters the Hells. This drive by the devils attracts the attention of more demons from the Abyss, which pushes the front line back into Avernus. The process repeats itself time and time again. To the good fortune of the rest of the multiverse, almost all the battles in the Blood War take place in the Abyss and...