Blood War

In the Dungeons & Dragons fantasy roleplaying game, the Blood War is an eternal conflict between the fiends of the Lower planes. The Tanar'ri are the demonic forces of the Abyss, an evil plane of chaos. Representing the equally evil but lawful realm of Baator are the Baatezu, the dominant caste of devilkin. Neutral evil Yugoloths play both sides against the other. The Blood War has raged on since the Age Before Ages, a dark period when the Prime Material Plane had not fully developed. The conflict is massive, spanning entire planes of reality, and hosting an immeasurable number of fiends.

The Blood War concept was introduced as part of the new background for the outer planes in 1991's Monstrous Compendium Volume Outer Planes Appendix. The conflict is revealed as a bitter war of annihilation between the baatezu race and the tanar'ri; an absolute, all encompassing, and virtually eternal struggle.

The Blood War was thoroughly detailed in various books throughout the Planescape setting, particularly the 1996 boxed set Hellbound: The Blood War.

The 4th edition of D&D's Manual of the Planes updated the Blood War into a smoldering cold war that was formerly an all-out war.

...
Wikipedia