The Sin War was a secret struggle fought between the Great Evils through the Triune and Inarius through the Cathedral of Light, the goal being the allegiance of humanity in the Great Conflict. It was a war that would inexorably link Man to the war between angels and demons, even after the conflict's supposed end.
The war between the High Heavens and Burning Hells was an everlasting conflict and as such, a stalemate. Attacks on one side would be repaid by counterstrikes; legions of demons and angels were obliterated in the process, but without causing a significant breakthrough. It was Inarius, tactical advisor to the Angiris Council of the High Heavens, who would eventually grow tired of the constant planning, counterplanning, and even counter-counterplanning. With the other members of the Council, even Tyrael, refusing to see Inarius's "reason", he gathered like-minded individuals from both Heaven and Hell, and led them far away from the carnage. After stealing the primal Eye of Anu, the ancient Worldstone, Inarius created a realm he called Sanctuary, where the renegade angels and demons could live in peace. Inarius also fell in love with a like-minded demoness, Lilith. Inarius made certain his former masters - and enemies - would not find his refuge as he shielded the realm from outside interference.
From the union of angel and demon the nephalem were created. Inarius wanted to eradicate the nephalem or 'first born', whom he eyed as abominations, but it was Lilith who first saw their potential. She realized the nephalem had the potential to become more than both angel and demon, and wanted to harness their power for her own ends. To this end she murdered most (if not all) of the renegade angels and demons. For her transgressions, Inarius banished her to the Void. Not fully able to eradicate the nephalem, Inarius now sought to contain their abilities, manipulating the Worldstone to dampen their powers.
For a while all seemed right, but eventually both sides of the Conflict were bound to take notice of Sanctuary. It was the Burning Hells which first discovered the realm, because of the reckless actions of Jere Harash. The Vizjerei Clan had long sought to summon "spirits," but having discovered the existence of demons, they began to summon them frequently and in ever increasing numbers, hoping to learn more about the universe and magic. What they didn't know was that their actions had alerted Hell to the existence of Sanctuary. The demons took notice of the mortal inhabitants of Sanctuary and realized its strategic value. Control over Sanctuary and its mortal population could gain the decisive advantage in the Great Conflict. In order not to alert the High Heavens, the forces of Hell opted for the subtle approach. The Prime Evil Mephisto sent his son, Lucion, to the realm, where he established a cult - the Triune. This cult would win over humanity to its side, gradually turning them over the side of Evil. However Inarius noticed the intrusion, and created a counter-sect - the Cathedral of Light - that preached the teachings of the Light. And so the great game had begun: a secret war of subterfuge and manipulations, with humanity as its grand prize.
Course of the War
The rise of these two religions polarized Kehjan society as their ideological battle continued over the next decade. The status quo between the two bodies was maintained, as agents of both religions carried their messages to the people, building bases of power, constructing monuments, and winning the absolute loyalty of faithful supplicants. However, at the war's end, 3000 years before the darkening of Tristram, the precarious balance between the Triune and the Cathedral dissolved due to the return of Lilith, who once again sought to bring the power of the nephalem under her control. To this end, she weakened the dampening effect of the Worldstone and manipulated a simple farmer - Uldyssian - into removing the main obstacles: the Triune and the Cathedral. Uldyssian led a band of followers called the Edyrem against the two bodies, for they saw them as the source of Kehjan's strife. Uldyssian's followers also had access to their nephalem potential, allowed for by Uldyssian's alteration of the Worldstone's attunement, allowing the Edyrem's powers to grow.
Though this proved effective at first - Uldyssian and his nephalem banished Lucion and toppled the Triune - Lilith eventually lost control of the situation. Uldyssian managed to severely weaken Lilith, allowing Inarius to once again banish her to the Void.
As the nephalem now turned on the Cathedral and Inarius, the High Heavens finally discovered the mortal realm as well. With the Cathedral in ruins and Inarius defeated, Tyrael, reacting against Uldyssian's apparent injustice against Inarius, unleashed the Heavenly Host upon the mortal realm. In turn, the armies of Hell erupted from the ground, and a three-way battle began between the angels, demons, and Edyrem. Uldyssian released his full power, driving the armies of Heaven and Hell back. However, in doing so, he realized that the untamed nephalem energies were threatening to destroy the world. It was too much power gained too quickly, and he could feel himself losing his humanity to his nephalem birthright. Thus, he brought the raging energies into himself and released them one last time, the release of said energies resetting the Worldstone and stripping the Edyrem of their abilities, but at the cost of his own being. In the end, Uldyssian had chosen humanity over godhood and sacrificed himself so that humanity might survive. So it was that he sacrificed himself.
In the aftermath of Uldyssian's sacrificed, the Angiris Council and Mephisto convened in order to decide the fate of the mortal realm. Though the Council was torn over a decision whether to spare the mortals or to eradicate them, they eventually voted for Sanctuary's continued existence, Tyrael himself casting the deciding vote, so moved was he by Uldyssian's sacrifice. In exchange for the renegade Inarius, Mephisto proposed a truce: the forces of Hell would no longer interfere on the mortal realm. Humanity was left to its own devices and left out of the Great Conflict.
A condition of Sanctuary's continued existence was a mind wipe of sorts for the surviving Edyrem. None were to remember the events of the war, so that humanity could choose between light and dark from an unbiased perspective; none would know of the existence of demons, angels, or nephalem. As far as most knew, the lives that had been lost in the conflict were the result of a plague, a lie that the altered memories of the Edyrem reinforced. People knew that battles had been fought, but that demons and angels had been involved, of that, they were ignorant. The Worldstone was altered so that yet again, humanity was prevented from developing its latent nephalem abilities. There were exceptions however, including Mendeln and the Mage Clans, who enforced new rules to ensure no demon would ever be summoned into Sanctuary again, and that Heaven and Hell would stay out of mortal affairs forever. The general population of Kehjan believed the Sin War to be nothing more than a clash of faiths. They went so far to rename their land as "Kehjistan" as a means to separate themselves from the conflict, and popular sentiment swung in favor of the mages rather than religious bodies, thus heralding the Age of Magic, However, some retained their memories of the war, and stories of the Sin War were passed down through subsequent generations.
Eventually, the Mage Clans' decree was breached, which led to the Mage Clan Wars. In turn, the Prime Evils refused to give up from their goal of converting humanity to their cause, and would return to plague the mortal realm in the Dark Exile.
Thousands of years later after the Sin War's end, Deckard Cain would write that, in a sense, the Sin War never ended. The conflict itself was testament to the power that human beings carried within them, and that the Sin War was a conflict that raged in human hearts long after the war itself ended.
The Sin War as is currently presented in Diablo lore stems primarily from its portrayal in The Sin War Trilogy of novels and subsequent lore. Prior to this, the Sin War was treated in a different light. Two key differences are its origins, and indications that the war did not have a genuine end, along with a different timeframe. These can be treated as retcons and/or inaccurate history as per the mind wipe enforced on the Edyrem.
The Sin War was first mentioned in the manual for the first Diablo game, detailed in the Librarius Ex Horadrim. Here, it is stated that Heaven and Hell became aware of the mortal realm simultaneously, and that both were content to see what path humanity would take. Hell discovered that mortals responded much more to brute force rather than subtle coercion and so began to terrorize Man into submission. Heaven fought to defend humanity against this demonic oppression, but all too often their austere methods and severe punishments succeeded only in alienating those whom they sought to protect.
The violent battles of the Sin War occurred very often, but they were seldom witnessed by the prying eyes of Man. Only a few "enlightened" souls were aware of the supernatural beings that walked amongst the huddled masses of humanity. Powerful mortals arose and accepted the challenge of the Sin War, allying themselves with both sides in the Great Conflict. The legendary deeds of these great mortal warriors served to earn both the respect and hatred of the netherworlds. Although the lesser demons kneeled before those possessing power and strength, they also cursed the very existence of mortal man. Many of these fiends believed that the deadlock brought about by the emergence of Man was a perverse offense to their "higher" role in the great scheme of things. It was this state of affairs that led to the Mage Clan Wars.
The origin of the war's name was also altered in the trilogy. According to The Realms Beyond (as found in the first game in the series), the term "Sin War" was a term used by Diablo to describe the battle for Sanctuary when conversing with Lazarus. In contrast, the Sin War Trilogy and Book of Cain establish that the term was in use long beforehand, not long after the war's end.
Timeframe and Continuation
The Sin War Trilogy and subsequent lore establish that the Sin War, as an actual event, distinctly came to an end with the pact between Mephisto and the Angiris Council. Prior to this however, the indication was that not only had the Sin War never ended, but had a different timeframe as well:
- In Diablo II, it is mentioned that the ruins in Aranoch were made as such, a thousand years ago during the Sin War, in the 3rd century. This conflicts with the novel trilogy which establishes that the war ended 3000 years ago.
- In Demonsbane, it is mentioned that the destruction of the demon Assur in the year 302 was one of the most significant early victories of the Sin War. 302 (early 4th century) matches up with the Aranoch figure above roughly. Again however, the timeframe is significantly different from the 3000 year figure.
- In The Black Road, it is mentioned that the Sin War lasted centuries, though does treat it as being a separate event from the timeframe of Diablo I.
- In Diablo, Adria mentions that securing the Anvil of Fury would steer the course of the Sin War towards the cause of Light. This matches the original lore that the Sin War was a continuous conflict.