The Blood War

Throughout history, the teeming hordes of The Infinite Layers of The Abyss  and the strictly regimented legions of The Nine Hells of Baator  have battled for supremacy in the cosmos. In the mortal world, the scant few scholars, arcanists, and adventurers who know the conflict for what it is refer to it as the Blood War.
The fighting takes place across the Lower Planes, on the Prime Material Plane, and anywhere else that demons and devils might congregate. From time to time, demons spill out of the Abyss to invade Avernus, the uppermost layer of the Nine Hells. While the devils defend their home turf, they also make strikes against locations in the Abyss. Although the intensity of the conflict waxes and wanes, and the front lines of the war can shift drastically, a moment never goes by when demons and devils aren’t battling each other somewhere in the multiverse.

The Great Dance

The battle lines in the Blood War undulate like a writhing snake. Each time one side gains an advantage, invariably its lines weaken somewhere else. Demons enter the Nine Hells by following the River Styx from the Abyss into Avernus. By hiring skilled merrenoloth pilots to navigate the river, a Demon Lord of the Abyss can transport an invasion force of enormous size.
Mimicking a complex dance, the two sides shift their attacks and trade positions with each passing day. About once every thousand years, the demons close in on the lower reaches of the Styx and portals leading deeper into the Hells. Invariably, the legions of the Nine Hells boil up from below to repel the invaders out of Avernus, back to the juncture where the Styx enters the Hells. This drive by the devils attracts the attention of more demons from the Abyss, which pushes the front line back into Avernus. The process repeats itself time and time again.
To the good fortune of the rest of the multiverse, almost all the battles in the Blood War take place in the Abyss and the Nine Hells. Whether by cosmic chance or the design of some unknown power, the dark waters of the Styx provide passage between the two planes, but pathways to other realms are at best fleeting and unreliable. Despite the difficulty of escaping the Lower Planes, combatants on both sides find their way to the Material Plane and other realms from time to time. Although the conflicts on these other planes are little more than skirmishes in the Blood War, even a small number of demons and devils can wreak havoc and bring destruction wherever they see fit to do battle.

Mortals and Minions

Devils and demons are far from the only combatants in the Blood War. Both sides exploit the Prime Material Plane's most abundant resource — mortal creatures, whose bodies and souls are both useful to the cause.
Devils constantly strive to recruit mortals into their ranks by offering them rewards in return for their service. While they live, these cultists carry out the wishes of their archdevil masters, whether raiding an enemy outpost or gathering more members for the cult. When a cultist dies, its soul emerges in the Nine Hells and becomes another of the Blood War’s immortal soldiers. Most of the evil souls consigned to an afterlife in the Nine Hells become lemures, which make up the vast majority of the hellish forces, but some mortal recruits who willingly accept a contract offer from a powerful devil can arrive as a lesser devil.
Demons generally have no regard for mortal souls and do not solicit them, but living creatures do have their uses. Groups of corrupted cultists dedicated to one of the demon lords exist all across the realms of the material world. For as long as these mortals do the bidding of their lord, they are allowed to live. From a demon’s perspective, all other living creatures are nothing more than sheep ripe for slaughter, and demonic cultists share this view. These fanatics don’t hesitate to slay other mortals if given the chance — and if their victims happen to be in league with devils, so much the better.
A casual observer might suppose that two forces of evil bent on exterminating each other would be an advantageous situation for the forces of good, but the combatants in the Blood War have no regard for collateral damage — and on the Material Plane, they can cause a lot of it. If agents of Asmodeus discover a thriving demon cult in a city, they might deal with the threat by starting a fire that not only destroys the cult but burns through several neighborhoods and kills hundreds of innocents. A demon might unleash a plague to kill every person in a town, just for the sake of claiming from its library an old book containing a map to a lost artifact. The fiends on both sides of the Blood War take the path of least resistance to their goals, heedless of consequences that don’t affect them.

Agents of Treachery

Both sides in the Blood War employ spies and soldiers drawn from the ranks of other evil creatures of the planes. The devils are generally more successful at this tactic because of the discipline they can bring to bear on these ostensible allies. Even so, powerful, intelligent demon lords such as Graz'zt can also force their agents to do their bidding under threat of annihilation.
As creatures that don’t favor either cause, because they care nothing for the philosophical concerns of law and chaos, other fiends including incubi, succubi, and night hags work for whichever side offers the best compensation. Demons use them as insurrectionists in the Nine Hells, inciting rebellion and defiance. The devils employ such creatures as scouts, who use their magic and other abilities to navigate the Abyss and gain intelligence about the demons’ activities. Adventurers hired by an agent of the Nine Hells to make a foray into the Abyss are customarily guided by such a mercenary.

The Search for Secrets

The Blood War has all the characteristics of an eternal stalemate, in part because the two sides are so familiar with each other. Every time the Abyss belches forth some new variety of horror, the disciplined and well-trained legions of the Nine Hells reorganize, rally, and counterattack. The devils continually dream up variations on their attack strategies, only to be checked by the overwhelming chaotic force of the Abyss. Little true advancement occurs under the angry red sky of Avernus or in any other realm where the forces clash.
The leaders of each side recognize that the introduction of some unexpected factor could permanently affect the balance of power. Accordingly, demons and devils constantly send their agents across the planes in search of artifacts, powerful creatures to recruit, and other resources that could lead to a key advantage in the war. Adventurers of great repute might get involved in such a quest, either as unwitting pawns or as an independent force pursuing its own ends.

Cosmic Battlefield

The Blood War rages along the length of the River Styx. Direct confrontations between demons and devils erupt along its banks, making any of the Lower Planes the Styx touches a potential battleground.
The devils view fighting demons on Avernus as a net benefit for their cause. Although most devils slain there are destroyed forever, ready access to supplies and support from the Nine Hells tilts the tide heavily against invading demons. Also, the prospect of a permanent death compels the devils fighting on Avernus to maintain the utmost readiness and coordination. Wounded devils retreat, knowing that fresh reinforcements can continue the fight. Hordes of lemures, devils that are permanently destroyed only if subjected to holy energies, are used to blunt demonic incursions. The terrain of Avernus is thoroughly mapped and festooned with ambush points, strongholds, and other defensive measures.
More important, demons that are slain and sent back to the Abyss return to their chaotic wanderings in that realm. A demon lord is thus hard pressed to keep a horde cohesive as it takes casualties. If the devils extended a tenacious defense out to the planes beyond Avernus, they could keep the demons away from Hell’s doorstep, but such a strategy would place a great strain on supplies, reinforcements, and unit integrity. Although the devils killed in such places would recover, their weapons, armor, and other materials would remain lost.

The Devilish Point of View

My legions are the only thing standing between your precious Seven Heavens and the bottomless hunger of the Abyss. I did not fall into the clutches of evil. I rose to shoulder a cosmic burden.

— Zariel, Archduchess of Avernus,

former angel of Celestia

Although the basic facts of the Blood War aren’t subject to debate, a host of theories exist that attempt to explain why the conflict erupted in the first place. The devils fight as a matter of pride mixed with burning resentment for a cosmic order that refuses to acknowledge their role as overseers of the multiverse.
To the devils’ way of thinking, the Nine Hells are the front line in the demons’ war against all of creation, and the fate of the cosmos depends on the devils’ ability to blunt the invasion and send the demons back to the Abyss.
In fact, devils believe that the demons would have long ago swept across the multiverse but for the unflinching rule of law that underlies the actions of the infernal leaders and their armies. If necessary, a general must not hesitate to sacrifice entire legions to stall an enemy advance or punch a hole through an enemy phalanx. Iron discipline combined with a ruthless focus on victory at any cost is what fuels the devils’ war machine.
Every devil takes a grim pride in its service in the Blood War. Talk to one for a short time, and it will reveal the scars earned during its time in Avernus and tell tales of the great battles it has seen. Talk to one for a little longer, and it will inform you that you are alive only because of the devils’ heroic efforts. If there was truly justice in the world, it would tell you, the multiverse would bend its knee to the Nine Hells in thanks for the devils’ role in preserving the cosmos.

The Demonic Point of View

Why do we descend upon the domain of our hated enemies? Because that’s where the fighting is!

— Zaadaaak, demon of the Abyss

Most other creatures believe demons to be little more than mindless engines of destruction and violence. If that supposition were true, the Blood War should have ended long ago, since the tactical and strategic genius of the archdevils would have made quick work of the hordes that erupt from the Abyss. In fact, even though demons are quintessentially chaotic, their evilness combines with that outlook to produce a fiendish, subtle shrewdness. Demon lords and other powerful entities exert control over their armies that enables the forces to accomplish goals that would be beyond the capabilities of a leaderless mob. To the demons that have intelligence enough to appreciate their role in it, the Blood War offers an endless source of diversion underscored by resentment of a cosmic order that refuses to admit their superiority.
As the devils have surmised, the demons invade Avernus because it is easily accessible from the Abyss. But some sages and demonologists maintain that even if that were not the case, the Nine Hells would still be the demons’ prime target. If the demons sent out a large force to invade a different plane, this theory holds, the devils would be able to marshal allies from across all the planes to rise up against the Abyss and end its threat forever. Instead, as long as the demons focus most of their efforts on the doorstep of the Nine Hells, realms such as The Seven Heavens of Mount Celestia are loath to commit aid to defend a plane that is inimical to everything they stand for by aiding the devils directly.
Therefore, the demons assault the Nine Hells over and over not only because it is the greatest threat to their dominion, but also because striking in force anywhere else would play into the devils’ hands.

The Balance

The demons and the devils both foresee their own versions of the future of the multiverse — a cosmos in which one side or the other triumphs in the Blood War and rules for the rest of eternity. A third point of view exists, held by those who take both sides at their word and strive to make sure that neither outcome ever comes to pass.
The adherents of this viewpoint refer to the concept they espouse as the Balance, and they seek to maintain equilibrium across the cosmos above all. Mordenkainen and his compatriots are among its most notable devotees. Since a true appreciation of the Balance and its implications requires a grasp of events on a cosmic level, very few people or entities fully embrace the idea. Those few, however, make up a formidable force that can mix martial and magical power to keep the extremists of the Blood War in check.
To an outside observer, a disciple of the Balance might act cruelly or heartlessly one day, and benevolent and caring the next. A follower might aid in consigning one city to domination by a devil cult while driving demonic cultists from another. For the sake of the Balance, the cosmos must remain in a static state where neither demon nor devil can permanently gain the upper hand.
Keepers of the Balance sometimes resort to strategies that seem inexplicable to those who don’t grasp the whole situation. A crusading paladin might be discouraged from seeking out and defeating Demogorgon, because doing so could weaken the Abyss enough to allow the forces of the Nine Hells to gain a firm foothold there. An adherent of the Balance might vie with a great arcanist as both search for knowledge of a ritual that would cripple Asmodeus for a short time. The arcanist correctly believes that performing the ritual would free a world of the Material Plane from the devils’ taint, but doesn’t appreciate that it also would bring a grievous setback to the forces that had been keeping the demons from overrunning Avernus.
Those who vow to maintain the Balance make enemies wherever they go, because their full reasons for acting as they do aren’t always apparent. The fact that some of these adherents embrace the Balance to further their personal quests for power makes it impossible to count on them as allies with complete certainty.

Conflict Type

War

Battlefield Type

Extraplanar

Location

Avernus