Ultimately, the Heavens and the Hells learned of Sanctuary's existence, and clashed across the world in the horrific Sin War. After the fighting was brought to a standstill, both sides agreed to cease their battles on Sanctuary, leaving mankind to its fate. Yet, Diablo, Mephisto, and Baal—the Prime Evils and leaders of the Burning Hells—plotted to use humans as tools of war.
This goal bred dissent among lesser demons, who revolted. Hell's legions were devastated, but they'd exiled the Prime Evils to Sanctuary. The archangel Tyrael, one of Heaven's leaders, noticed the Evils in the human world and covertly formed the Horadrim, a group of magi, to hunt the Primes. Armed with soulstones—relics that could contain the Primes' essences—the Horadrim imprisoned their quarry, and hid the souls of the Three throughout Sanctuary.