The full story of Diablo: Timeline, lore, and history summarized

Diablo Lore: The Dark Exile

So the truce between Heaven and Hell went swimmingly, and the Eternal Conflict had finally come to a conclusion, right? Well demons aren't exactly known for their honour when it comes to deals, so it's not exactly a surprise that the Prime Evils went straight back to plotting how to return to Sanctuary and wreak havoc, or at least get humanity to side with them against Heaven and ultimately win the Eternal Conflict (yes this again).

The Lesser Evils, weren't so enamored with this plotting. The despised the obsession over humans and didn't understand the fascination as they believed Demons to be superior in every way, having not experienced the might of Uldyssian themselves. The Lesser Evils wanted to continue to fight the High Heavens and viewed all this Sanctuary lark as an irritating side quest in the ultimate goal.

The four Lesser Evils banded together, to wage a rebellion against the Primes, turning their followers against them and eventually winning and banishing them to Sanctuary. Azmodan, the tactician, believed that the Prime Evil's presence in Sanctuary would turn Heaven's interest back toward humans, and they would leave the Gate of Heaven unguarded.

Whilst the Lesser Evils celebrated their victory, so to did the Prime Evils as it had been their plan all along. By being overthrown and banished to Sanctuary they themselves hadn't violated the terms of the truce. 

The Three Prime Evils ravaged Sanctuary, taking on human hosts to corrupt humanity, feeding upon the worst parts of men across the Eastern regions, causing pain and suffering wherever they interfered.

Eventually, their presence was noticed by Tyrael, and knowing how shaky a foundation the truce was made on, he decided to take matters into his own hands without alerting the Angiris Council. Gathering members from the Mage Clans, he formed the Horadrim. The Horadrim were an order of crusaders standing for justice and light, bought together to combat the Prime Evils specifically. Members of the Horadric order included Zoltan Kulle, Tal Rasha and Jered Cain. Names you will become familiar with if you do go on to play the games.

Tyrael had prepared special artefacts using shards of the World Stone in which to capture the Prime Evils. These Soul Stones were given to the Horadrim to aid in their quest to capture the three brothers and imprison their essence in each of the stones.

Mephisto was the first to be captured in the lands of Kehjistan, his soulstone was then guarded in Kurast by a religious order, the Zakarum.

Baal was the next to be captured, however, the ensuing battle led to the destruction of his soulstone. Having only a small shard in which to capture him which would only work in the short term, Tal Rasha selflessly volunteered to sacrifice his human body to be a vessel for capture and using the remains of the stone he would battle eternally to keep Baal in captivity within himself. Tal Rasha was then kept in unbreakable chains in a tomb in the Canyon of the Magi.

Diablo was the last to be found, 15 years after their quest first began. In Khanduras a final showdown took place between Diablo and the Horadrim that nearly wiped out the order. Diablo was imprisoned in a soulstone and this was hidden in a cave, found in a labyrinth near the River Talsande. The Horadric order built a monastery to guard the labyrinth lest Diablo ever be discovered and released.

Villages and other settlements sprouted up around the monastery over the years, and eventually the town of Tristram. The monastery itself would come to be known as Tristram Cathedral, and Diablo would remain trapped there for two centuries as humans forgot about the existence of the Prime Evils, and even the Horadrim themselves.