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.
The Blood War is 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 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.
However, the fiendish codex Tyrants of the Nine Hells claims a different perspective on the origins of the Blood war. The mythologies in the codex claim that the seeds of the Blood War lay in the cradle of the cosmos itself; "In the beginning—and even before—chaos was all that existed. Out of it came demons—the living manifestations of chaos...". It claims that the chaos was intolerable to the universe, and thus Law arose to fight it. Initially, the primordial lords of order battled the demons themselves, however after inventing numbers and time, they came to understand the demons were infinite in number. Knowing this, they grew weary of endless conflict, and yearned to build worlds and foster races. Thus, they created winged warriors dubbed (in later terms) 'angels' to fight the demons for them. The bravest, toughest and fiercest angels were headed by one named 'Asmodeus'. They were the champions of combat and killed far more demons than even their masters. However, as the eons wore on, to better combat the demons Asmodeus and his angels took on some of the fierce and terrible traits of the demons. Soon they were ugly, monstrous, barbed, and armored beings of combat, and the other angels and gods shunned them. Rightly pointing out that they were merely fulfilling their duties, indeed superbly, legally Asmodeus and his kindred could not be banished. In a series of events that eventually led to the pact primeval, Asmodeus and his angels, now dubbed 'devils', were either granted (or exiled to, depending on perspective) a realm of their own to take the unpleasant combat of what was essentially the blood war away from the realms of pure Law which held no concept of Good or Evil. This realm was Baator, the nine hells, and thus formed the 'Blood War'.
However, as detailed as the devils' mythology of the origin of the Blood War happens to be, it stands at odds with other sources, and especially at odds with sources older than their own race. In true Baatorian fashion, their mythology is a byzantine masterpiece of historical revisionism and state-mandated "truth", which might have little factual basis from an objective viewpoint, and the second Fiendish Codex openly admits to this, though to the devils themselves, the difference might be meaningless. The devils' mythology conveniently omits that the plane of Baator was not an empty plane when they arrived there, and in fact they conquered its original inhabitants, the Ancient Baatorians. Additionally, while their mythology holds that Law arose to battle the primordial, pre-existent forces of Chaos, their original enemies from the Abyss, the obyriths, were not actually native to that plane, but were the creations of an even older race of original creator fiends, unaligned with either Law or Chaos, whose presence is curiously absent from the devils' historical writings.
Despite millennia after millennia of constant strife, no side has yet been able to gain a definitive, permanent advantage over the other. Despite their vast differences, the tanar'ri and the baatezu are surprisingly balanced combatants on the fields of the Blood War. 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.
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, while the female demon lord J’zzalshrak, called The Errant General, has Blood War campaigns as her portfolio.
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.
- Grubb, Jeff, David Noonan, and Bruce R. Cordell. Manual of the Planes. Renton, WA: Wizards of the Coast, 2001.
- Jacobs, James, Erik Mona, and Ed Stark. Fiendish Codex I: Hordes of the Abyss. Renton, WA: Wizards of the Coast, 2006.
- Laws, Robin D, and Robert J. Schwalb. Fiendish Codex II: Tyrants of the Nine Hells. Renton, WA: Wizards of the Coast, 2006.