The protagonist of Diablo 1 was really the Prime Evils pawn
The hero of Diablo 1 ventured into the depths of the cathedral to slay Diablo (canonically, Leorics warrior son Aidan was the one to defeat the big red guy). But even after being trapped in the soulstone once more, Diablo was able to corrupt the heros mind, convincing him to plunge the soulstone directly into his forehead. Instead of banishing Diablo back to the void of the soulstone, the hero ended up giving him an even stronger body to possess.
as was Marius, the narrator of Diablo II
This poor sap got caught in a battle between worlds, when all he wanted to do was smoke his pipe. Marius was relaxing in the same tavern as Diablo 1s hero (now referred to as the Wanderer) when the Wanderer had a bit of a demonic-possession episode, killing everyone around him save Marius. Stunned at the catastrophe around him, Marius becomes further and further unhinged, which isnt helped by the fact that the Wanderer beckons to join him on his journey. Over the course of Diablo II, an increasingly-insane Marius obeys the orders from Diablo and Tyrael alike - but he dooms humanity when he mistakenly relinquishes Baals soulstone to the Prime Evil himself, thinking him to be Tyrael. Mephisto and Diablo may have been slain and their soulstones destroyed, but that still left one loose end.
Mount Arreat was the last stand for humankind
Now in possession of the one object that could defeat him, Baal turned his sights to the Worldstone, the source of Sanctuarys protection and a gateway to other realms. It remained hidden in the mountains of Mount Arreat, where barbarian settlers had made a home for themselves called Harrogath. Baal sieged the small city with a massive army, but the heroes of D2s expansion were able to push them back and track down Baal. Unfortunately, Baal managed to corrupt the Worldstone before his death, and drastic measures had to be taken
Its been 20 years since Tyrael destroyed the Worldstone
Taking matters into his own hands, Tyrael made the critical decision to nip the pressing problem in the bud. Warping to the Worldstone Keep after Baal falls, he transports the party of heroes back to Harrogath in preparation to enact some angelic demolition. Knowing that the Worldstones been tainted beyond repair, and could threaten all life in Sanctuary, Tyrael heaves his sword of light into the crystal, shattering it into thousands of pieces. That was 20 years before the events of Diablo III, and nobodys heard from Tyrael since.
Deckard Cain is getting too old for this $#!@
Having studied the heroics of his magi ancestors, Deckard Cain is an expert on all things related to the Horadrim, including D2s Horadric Cube. This frail old historian winds up in the most unlikely of place hes there to help in Diablos I, II, and III, whether via giving quests, identifying items, or acting as an interpreter for Tyraels vague commands. Also, he sounds a little bit like an elderly Sean Connery. Hell play a key role in Diablo III, as his adopted daughter Leah seems to be getting quite a few demonic visions lately
Evils making a comeback
Leah takes after her Uncle Deckard, studying the relics of the Horadrim and researching the humans vs. demons battles of past generations. Knowing that the Lesser Evils Azmodan and Belial are still waging war in Hell, she seeks to imprison them using a fourth soulstone shes recovered: the Black Soulstone. (Diablos was red, Mephistos was blue, and Baals was yellow, in case you were wondering). But recovering the artifact has alerted Azmodan to its presence, and he seeks to hunt Leah down and destroy the Black Soulstone before it can be used against him. Theres no doubt in our minds that Azmodan and Belial will have a bone to pick with our Diablo III character
Want more lore?
Hop over to our Diablo III review to get our final verdict on the long-awaited game. Need to get up to speed in other games? Check out the 21 must-know facts about the Mass Effect universe to get the skinny on Commander Shepards storied military career, or get even more hyped for Diablo's return with our 13 reasons to get excited for Diablo III.