"From the very moment of the existence, the High Heavens and the Burning Hells clashed in the Eternal Conflict, an apocalyptic war for dominion over all creation."(src)
"There is a war that rages on even now, beyond the fields that we know--between the utopian kingdoms of the high heavens and the chaotic pits of the burning hells. This war is known as the great conflict, and it has raged and burned longer than any of the stars in the sky. Neither side ever gains sway for long as the forces of light and darkness constantly vie for control over all creation."— The Great Conflict (excerpt)(src)
The war was most commonly fought in the realm of Pandemonium, as both sides fought for control of the Worldstone. It is said that whoever controlled the Worldstone had the ability to shape reality and create life and worlds almost without restriction. Angels used the stone to create worlds of perfect order in line with their ideals. Demons used it to create engines of annihilation and worlds of destruction, terror, and hatred. However, none of the worlds, regardless of their maker, ever flourished, and were doomed to wither and die due to the war.
At some point, the archangel Tyrael ordered a bastion to be built around the Worldstone. This stronghold came to be known as the Pandemonium Fortress. It changed hands many times over the course of the conflict and as such, its structural and metaphysical traits of both demons and angels.
At times, the war would reach Heaven and Hell itself. The Diamond Gates of the High Heavens were assailed at least five times, but the gates themselves were never breached. Likewise, angels assaulted Hell. One such assault was made against Mephisto himself in his own realm, and another against the Hellforge himself, where Izual was lost. But while the Great Conflict burned hotter and longer than any of the stars in the sky, neither side could gain dominion over the other for long. Both factions sought some way to turn the tides of the war to their favor.
Disappearance of the WorldstoneEdit
While the armies of Heaven and Hell clashed, Inarius and Lilith were disgruntled and disillusioned by this unending conflict, both seeking a way out of it. They led a renegade group of like-minded angels and demons to steal the Worldstone and used it to create a world of their own, where angels and demons can co-exist peacefully away from the Eternal Conflict and soon bore the nephalem.
The disappearance of the Worldstone soon caught the attention of both Heaven and Hell, puzzled on how such a powerful item to create worlds disappeared without a trace.
The Sin WarEdit
Eventually the forces of Hell learned about Sanctuary. Not wanting to attract Heaven's attention, they worked in secret to try and corrupt humanity via the Triune reasoning that their potential would allow them to finally break the deadlock in the Great Confict. They were opposed by Inarius, although both Inarius and Lucion were defeated by Uldyssian. The High Heavens found out about Sanctuary and moved to destroy it. Eventually Uldyssian's heroic sacrifice drove both armies back and resealed humanity's potential. After the Heaven's voted to spare sanctuary a truce was reached with the Burning Hells; both factions would respect Sanctuary's right to exist. However, the Prime Evils had no intention of honoring the truce.
- An event called "Eternal Conflict" was held in Heroes of the Storm, corresponding to an influx of content based on the Diablo universe.
- A "Second Great Conflict" was referenced in the Diablo Writer's Bible in 2000, where paragraphs outlined that in a battle between order and chaos, powerful heroes would emerge. This bears resemblance to the rise of the Nephalem in the released version of Diablo III, though dialogue within the game places the game's events within the scope of the same Eternal Conflict, rather than a separate one.