Blood War

The Blood War is an eternal conflict between the fiends of the Lower Planes. The Tanar'ri lead the demonic forces of the chaotic evil plane of The Abyss. Representing the equally evil but lawful realm of Baator are the Baatezu, the dominant caste of devilkin. The Blood War has raged on since the Age Before Ages, a dark time when the Prime Material Plane had not fully developed yet. The conflict is massive, spanning entire planes of reality and hosting an almost infinite number of fiends.

Motivations
There have been thories that dictate the Blood War is a simple offshoot of the primordial wars between law and chaos. The demons and devils, in keeping with their sadistic and violent natures, simply would not cease their fight, even after the rest of the multiverse had grown more tolerant.