From Diablo Wiki
While this depicts "The Sin War", other sources mentions additional times when the Prime Evils roamed Sanctuary, and the term Sin War seems now more related to demon/human/angel battles on Sanctuary in general.
The Sin Wars were stopped when the three were imprisoned in Soulstones, and started raging (in small scale) when they were released. For the last 10 years, it has been calm again, but somewhere someone is planning something. It most definitely has something to do with a certain meteor.
Diablo I Manual
Diablo I Manual states excerpt called "The Sin War":
- 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 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 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 mageclan 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.
Details About the Sin Wars
Ever since Sanctuary was created, a massive amount of energy has existed here. The original nephalem were allowed to live under the condition that the Worldstone would lower their power, and they would not remember that they used to wield it.
Both the Prime Evils and the Angiris Council seeks to control Sanctuary. Diablo has expressed an interest in being lord over the mortal realm, but that is hardly the main reason the Burning Hells halted their wars with the High Heavens.
The greatly powerful beings of both Heaven and Hell find themselves greatly reduced in strength while in the mortal realm. They can't draw or unleash unlimited powers from their home realms. However, power given to them by mortal souls either as allies, worshippers or slaves can be used as a source of energy that is also potent in other realms. To dominate the mortal realm is to dominate a massive source of power that the opposing side can not access and giving a massive edge in the Great Conflict. This is why the battles of Heaven and Hell rages on Sanctuary, as human souls are their prize.
Izual fell into Darkness and saw the Soulstones as a way for the Three to channel vast amounts of power directly into Sanctuary from their home realms and allowing Hell's forces to take full control. To gain possession of the Soulstones, they needed to lure the Angels to use them against them. Since a full confrontation likely would destroy the mortal realm, destroying the possibility of gaining more power, the Three stealthily manipulated the Lesser Evils into sending them on the Dark Exile. To be exiled in a weaker spirit-form would not wake such a great suspicion in the High Heavens.
Falling for the trick, the Angels withdrew their forces from the mortal realm as they assumed the Lesser Evils would launch an all-out attack against the High Heavens. They left mankind to fend for itself. Should mankind be able to defend itself they might be worthy allies, and should they fail they were obviously nothing mroe than a liability in the Great Conflict.
While empowering their fortifications, the Angels of Heaven could not ignore the opportunity to capture the Three within the Soulstones. Thus the Archangel named Tyrael bestowed the three Soulstones and their secrets upon the Horadrim, an order of mortal magi.
The Three thought they were in control, and were distraught when Izual's warnings to them about becoming weakened came true. Without a physical vessel to contain their spiritual-form, they were unable to hold onto the massive supply of energy from Hell. To just manifest in the mortal realm required heavy expenditures. Bleeding power and severed from their source of energy the Three were forced to possess mortals to stop further waning of their powers.
When they did this they realised that not only did it stop the expenditure of their energy, but they could use the body to store energy stolen from human souls. The amount of energy that could be stored seemed to vary depending on the strength of the mortal's mind, body and spirit. The possessed was also also warped into a hideous parody of each of the Three's own demonic physical appearance. They ravaged the lands of the mortal realm and consumed human souls to amass power.
Unexpectedly, the Horadrim were more powerful than they could have imagined, and as they defeated Mephisto in Kehjistan and imprisoned him, he was not strong enough to warp the Soulstone for his own use and too weak to even break free.