Blood War

The Blood War is an ancient conflict that once raged 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 played both sides against the other. The Blood War had raged on since the Age Before Ages, a dark time when the Prime Material Plane had not fully developed. The conflict was 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 liked to entertain the notion that the War was their own personal experiment into the nature of evil, one which they created and would ultimately end on their own terms. Of course the other fiends had their own notions about the war, colored by their particular alignments and racial propaganda. Some demons and devils seemed 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 paid the Blood War little concern, and if entities as ancient and formidable as they seem unconcerned, then the cause would most likely be a fruitless one.

Many lesser fiends actually embraced the War, for it staved away the cosmic boredom of being ageless. A few races survived 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 were bred by other fiends exclusively for the Blood War, including the Nessian war hounds and the milvorn. Even the yugoloths were given purpose by the Blood War; forever shifting from one side to the other, they as a race gained much of their wealth and power from the spoils of war and the high price of their services, though some (including the yugoloths themselves) believed this to be simply a facade over older and darker motivations.

The Fiendish Codex I: Hordes of the Abyss dictates that the Blood War was 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 had been able to gain even a definitive, permanent advantage over the other. Despite their vast differences, the tanar'ri and the baatezu were 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 could 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, deployed 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 have kept the Blood War fueled for an indefinite period of time.

One thing, however, is certain: had one side been able to eliminate all opposition, and thus gain control over the entirety of the Lower Planes, the multiverse as a whole would have been in great danger. With no enemies left in the Lower Planes, the fiendish rulers could then have turned 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 by their alignments demand by the needs of good and justice—the fiends, having no such compunctions or scruples, could easily have used the celestials' morality against them. In the past, agents of the heavenly races had 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 did not want the war to end.

There is a prophecy that says that the Blood War would 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.

Thus the seemingly eternal Blood War finally came to an end at the hands of the deified Archdevil Asmodeous; with the devils thus victorious in a manner of speaking.

High ranking officers
The Dark Eight are a coven of pit fiend generals who reside on the Ninth Layer of Hell, Nessus. They plotted and strategized the movements of Hell's armies. Bel, Lord of the First Layer, was once a mighty leader on the battlefields of the War, and used 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 did not take an active role in the Blood War. The sole exception from Baator was Bel, Lord of Avernus. The mighty pit fiend has strong ties to the Dark Eight, the pit fiend generals who oversaw all of Hell's involvement in the War; this relationship kept 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 had little concern for the Nine Hells. Asmodeus himself built a mighty force of devils within his fortress of Malsheem in wait of a cataclysmic battle he claimed would dwarf the Blood War.