Diablo I Manual
The coming of the Great Conflict to the mortal realm is known as the Sin War. Angels and Demons, disguising themselves while traveling amongst men, attempted to secretly lure mortals to their respective causes. Over time, the forces of Darkness discovered that mortals responded much more to brute force than to subtle coercion, and so began to terrorize Man into submission. The Angels 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.
This jealousy of Man led to harsh, atrocious acts of violence by the demons against the mortal realm. Some men learned of this deep hatred and used it against the denizens of the Underworld. One such mortal, Horazon the Summoner, delighted in summoning demons and then breaking them to his will. Horazon, along with his brother Bartuc, were members of the Eastern mage-clan known as the Vizjerei. This mystic clan studied the ways of demons and had catalogued their lore for generations. Empowered by this knowledge, Horazon was able to take the work of the Vizjerei and pervert it for his demented purposes. The denizens of Hell sought revenge against this bold mortal, but Horazon managed to keep himself well protected within his arcane sanctuary.
Bartuc, the brother of Horazon, was eventually lured to the side of Darkness. He was granted exceptional strength and longevity, and fought alongside the legions of Hell against the cursed Vizjerei, and eventually his own brother during the Sin War. Although Bartuc was renowned amongst the warriors of many realms, his dominance in battle came with a terrible price. An insatiable lust for mortal blood pervaded his every thought and deed. Bartuc soon became as fond of bathing in the blood of his enemies as he did of shedding it, and in time he came to be known only as the Warlord of Blood.
The Sin War is not a single great battle that was fought and now belongs to the past. The Sin War was, is, an age during which both Heaven and Hell shifted the focus of the great conflict to Sanctuary and came to a halt pending on which side humans will choose.
The Sin War seemingly ended with the overthrowing of the Three and their exile to the mortal plane, because at that point, the focus of the two sided of the great conflict was restored towards each other, at least in some part.
But Diablo names the Sin War the battle for Sanctuary and all who exist there, and announces that once he releases his brothers, “The Sin War will once again know the fury of the three.”
Something in Sanctuary is highly valued by the three Prime Evils. It is seen as the ultimate means in order to gain the upper hand over the forces of Heaven and win the Great conflict, and though what it is exactly might be disputed, there is no denying that it does exist.
The greater lords of Heaven and Hell found that their powers were greatly reduced while in the mortal realm. They are limited in the amount of power they can draw and unleash into the mortal realm from their respective home realm (the source of their greater powers). They learned that the soul energy of their human allies and worshipers could be used as a source of energy, not only in the mortal realm but in other realms as well. Dominance of the mortal realm would allow the controlling side to harness these energies from human souls (while denying the opposing force from access to this vast source of power) and grant them an edge against the enemy in the Great Conflict. Thus, the forces of Heaven and Hell began to struggle for control of the mortal realm with human souls as the prize.
Izual, who had by now fallen into the Darkness, suggested that the Soulstones might be manipulated by the Three in order to channel great amounts of power from Hell directly into the mortal realm, allowing the forces of Hell to seize control. In order to gain possession of the Soulstones, the Three lure the Angels into using the Soulstones against them. Realising that a direct incursion by the Three into the mortal realm might lead to a great confrontation between the forces of Heaven and Hell that had every chance of destroying the mortal realm, the Three secretly manipulated the events leading to the Dark Exile.
By allowing themselves to be exiled in spirit-form, therefore appearing as a lesser threat, the Three set the perfect stage for the use of the Soulstones against them by the forces of Heaven.
Learning that the revolution in Hell was set into motion by aggressive Demon-Lords, frustrated by the standstill in the Great Conflict, the Angels were certain that a mass assault upon the realms of Heaven by the forces of Hell was imminent. In preparation for the attack, the Angels withdrew their forces from the mortal realm, leaving mankind to fend for itself. The Angels of Heaven felt that the mankind's defense of their own world would be a way to prove themselves as worthy allies or a liability in the Great Conflict.
Although placing their main focus on the defense of their realms, the Angels of Heaven could not ignore the opportunity that was present to capture the Three within the Soulstones. Thus the Archangel Tyrael bestowed the three Soulstones and their secrets upon the Horadrim, an order of mortal magi.
Events seemed to be proceeding as the Three had planned. Yet, in their overweening pride and overconfidence, the Three failed to consider the loss of strength incurred by their exile in the mortal realm. The Three had laughed at Izual's warnings that they might become weakened enough to be at risk, sure in their arrogance that no Angels could ever overcome them even in such a state.
Without a physical vessel to contain their spiritual-form, they found themselves unable to hold onto the vast supply of energy from Hell. They found that the mere act of manifesting in the mortal realm required heavy expenditures of energy. Steadily losing power and severed from their greatest source of energy, the Three were forced into possessing mortals to prevent the further waning of their power.
Upon possessing a mortal they discovered that not only were they thus able to exist on the mortal realm without expenditure of their energy, but that the mortal vessel could be used to store energy stolen from human souls.
The amount of energy a mortal vessel held seemed to vary depending on the strength of the mortal's mind, body and spirit: The greater his strength, the more could be held. The possession warped the physical form of the human vessel into a hideous parody of each of the Three's own demonic physical appearance.
The Three ravaged the lands of the mortal realm, gorging on human souls to rebuild their power. When Mephisto was finally hunted down and trapped within a Soulstone, he was not strong enough to warp it for his own use and too weak to even break free.
Baal during his encounter with the Horadrim managed to shatter the Soulstone before it could be used against him. However the battle left Baal weakened and forced to flee. Diablo and Baal surmised that though the shattered Soulstone might no longer be strong enough for what the Three had originally intended. They decided instead to return to hell, once there to draw upon the Energies of their realm to rule again and lead the hordes of Hell to utterly destroy the mortal realm. Baal and Diablo then went their separate ways to re-gather the energy needed for this task.
When Baal felt he was powerful enough he confronted the party of Horadrim mages in possession of a Soulstone fragment, and who were hunting for him. Tal Rasha, an initiate of incredible will and magical power, led the Vizjerei, a group of Horadrim mages. Baal allowed himself to be drawn into the fragment of the Soulstone so he might attempt to twist it to his use. However Tal Rasha then bound Baal with his own soul by using himself as an extension of the Soulstone. Now a living Soulstone, Tal Rasha insisted he be buried and bound in the depths of a tomb beneath the great desert, the location of which was to be forgotten for all time.
Diablo was finally captured in the West by the Horadrim. It was found that in his weakened state to be unable to break free of the binding of the Soulstone. Nonetheless, his Soulstone was buried beneath Tristram's monastery and guarded by the Horadrim. However, over the years the guardians numbers and strength dwindled, and Diablo constantly tested the bounds of his imprisonment, slowly twisting the Soulstone to his needs. Like a slow malignant cancer he spread his evil presence into the Stone, warping its nature.
End of the War?
Now with the Worldstone destroyed, it is unknown if that goal is now forever lost to the forces of Hell or if it is now fair game for both sides, or maybe something else entirely.
What is certain is that since Diablo III must have humans fighting for their lives against the hordes of chaos lead by at least Diablo himself, then the battle for Sanctuary isn't over - the Sin War is far from being over.
Excerpt from the Realms Beyond
"All praises to Diablo - Lord of Terror and Survivor of the Dark Exile. When he awakened from his long slumber, my Lord and Master spoke to me of secrets that few mortals know. He told me the Kingdoms of the High Heavens and the Pits of the Burning Hells engage in an eternal war. He revealed the powers that have brought this discord to the realms of man. My Lord has named the battle for this world and all who exist here the Sin War."