Blood War

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 time when the Prime Material Plane had not fully developed. The conflict is massive, spanning entire planes of reality, and hosting an almost infinite number of fiends.

Motivations
The cause of the Blood War is unknown, but most believe it to be a remnant of the great war of Law against Chaos waged by the Wind Dukes of Aaqa against the forces of the Queen of Chaos. The yugoloths like to entertain the notion that the War is their own personal experiment into the nature of evil, one which they created and will ultimately end on their own terms. Of course the other fiends have their own notions about the war, colored by their particular alignments and racial propaganda. Some demons and devils seem to believe that their respective races could actually gain control over the other. However, the most powerful archdukes of Hell and the most cunning princes of the Abyss pay the Blood War little concern, and if entities as ancient and formidable as they seem unconcerned, then the cause is most likely a fruitless one.

Many lesser fiends actually embrace the War, for it staves away the cosmic boredom of being ageless. A few races survive on the carnage the Blood War produces; the armanites, centaur-like tanar'ri, must constantly battle with other creatures, for otherwise they will turn on one another; the molydei are another race of demons who prowl the Abyss, searching for deserters or rogue demons, whom they press-gang into fighting. Certain creatures are bred by other fiends exclusively for the Blood War, including the Nessian war hounds and the milvorn. Even the yugoloths are given purpose by the Blood War; forever shifting from one side to the other, they as a race gain much of their wealth and power from the spoils of war and the high price of their services, though some (including the yugoloths themselves) believe this is simply a facade over older and darker motivations.

The Fiendish Codex I: Hordes of the Abyss dictates that the Blood War is a simple offshoot of the primordial wars between law and chaos (it is unknown as to whether the lawful good-aligned archons and chaotic good aligned eladrins participated in the conflict). 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.

Outcome
Despite millennia after millennia of constant strife, no side has yet been able to gain even a definitive, permanent advantage over the other. Despite their vast differences, the tanar'ri and the baatezu are surprisingly balanced combatants. The chaotic denizens of the Abyss, while far more numerous than the devils, are, true to their alignment, constantly warring amongst themselves. They can contest the might of the Nine Hells only through sheer individual power, and their seemingly limitless, if unorganized and uncoordinated supply of warriors. The baatezu, on the other hand, deploy smaller numbers onto the field, but their warriors are regimented, well-trained and well-disciplined, all the while making incredible use of their generals' ruthless strategies. This violent balance could keep the Blood War fueled for an indefinite period of time.

One thing, however, is certain: were one side able to eliminate all opposition, and thus gain control over the entirety of the Lower Planes, the multiverse as a whole would be in great danger. With no enemies left in the Lower Planes, the fiendish rulers could then turn their attentions towards other worlds and planes, and it is likely that no force could hope to stop them. Even the celestial beings of the Upper Planes, formidable as they are, could face destruction at the hands of the tanar'ri or the baatezu, weighed down as their alignments demand by the needs of good and justice—the fiends, having no such compunctions or scruples, could easily use the celestials' morality against them. In the past, agents of the heavenly races have even planted powerful weapons or artifacts among the ranks of the demons or the devils. This appears to be a reliable indicator that even the beings of the Upper Planes do not want the war to end.

There is a prophecy that says that the Blood War will end when the Crawling City, a city on the plane of Gehenna that is home to millions of Yugoloths, directly enters the Blood War.

This proved untrue, as once Asmodeus regained his divinity by absorbing the divine power of Azuth he ended the blood war by pushing the Abyss to the bottom of the Elemental Chaos.

High ranking officers
The Dark Eight are a coven of pit fiend generals who reside on the Ninth Layer of Hell, Nessus. They plot and strategize the movements of Hell's armies. Bel, Lord of the First Layer, was once a mighty leader on the battlefields of the War, and now uses his influence with the Dark Eight to retain control of Avernus.

Baltazo, a grotesque minor Demon lord, was also a mighty general in the Blood War, but has since retired to the Plain of Infinite Portals.

Interestingly, the greatest powers of the Nine Hells and the Abyss do not take an active role in the Blood War. The sole exception from Baator is Bel, Lord of Avernus. The mighty pit fiend has strong ties to the Dark Eight, the pit fiend generals who oversee all of Hell's involvement in the War; this relationship keeps the relatively weak Bel safe from the other archdukes. The mightiest demon princes, such as Orcus, Graz'zt, and Demogorgon, constantly war with each other within the Abyss, and have little concern for the Nine Hells. Asmodeus himself builds a mighty force of devils within his fortress of Malsheem in wait of a cataclysmic battle he claims will dwarf the Blood War.