From Diablo Wiki
This is the timeline of the Diablo Universe.
Unless marked as speculation or a non-specific date, the information here is 100% canon. Any inconsistenceis with other documents can be seen as retcons made. Bashiok on Act 3 and Timeline Inconsistencies
Led by Azmodan and Belial, many of the Hells' lesser demons rise up against the Prime Evils in a rebellion that culminates in the Dark Exile. Diablo, Mephisto, and Baal are exiled to Sanctuary from the Burning Hells.
Diablo is finally captured by a group of Horadrim monks led by Jered Cain. The monks bury Diablo's Soulstone near the river Talsande in Khanduras, and a Horadric monastery with a network of catacombs is built over the burial spot.
The Horadrim, with no quests left to undertake, fade into history and legend.
- The eastern lord Leoric comes to Tristram, declares himself its king, and converts the old, long-unused Horadric monastery to a Zakarum cathedral. His advisor, Archbishop Lazarus, secretly releases Diablo from imprisonment.
- King Leoric descends into madness as he valiantly struggles to resist Diablo's possession.
- Increasingly deranged, King Leoric begins imprisoning all who question his authority and executing them as traitors. He declares war on the kingdom of Westmarch.
- Diablo kidnaps and possesses King Leoric's only son, Prince Albrecht.
- Lachdanan, captain of King Leoric's army, returns from the disastrous war with Westmarch and is forced to kill Leoric. The fallen monarch curses Lachdanan and his followers with his dying breath. Not long afterwards, Lachdanan and the king's guards are attacked by Leoric, resurrected as the Skeleton King, as they attempt to bury him.
- Archbishop Lazarus leads a group of villagers into the cathedral and abandons them to a bloodthirsty butcher demon. People begin fleeing Tristram in droves.
- A lone hero arrives in Tristram and defeats the bloated, fetid butcher demon.
- The hero kills Archbishop Lazarus and defeats the Skeleton King.
- Diablo is defeated by the hero, who then tries to contain Diablo within himself. Despite the hero's noble intentions, his valiant spirit quickly succumbs to Diablo's corruption. He becomes the Dark Wanderer, leaving Tristram behind shortly before legions of foul demons attack and destroy the town and its inhabitants.
- A group of heroes begins a quest to defeat Diablo and his brothers. The heroes defeat the demoness Andariel and journey east, following the Wanderer.
- The demon lord Duriel is defeated within the Tomb of Tal Rasha; it is discovered that Baal is already free from his Soulstone.
- The Dark Wanderer completes his transformation into Diablo; Mephisto is destroyed beneath the jungles of Kurast by the band of heroes.
- Diablo is defeated. His Soulstone is destroyed, along with Mephisto's, upon the Hellforge.
- Baal and his army march on Mount Arreat.
- The heroes follow Baal to Mount Arreat for the final conflict between man and the last of the Prime Evils. Although the heroes defeat Baal, his demonic army continues to advance.
- The archangel Tyrael discovers that Baal has succeeded in corrupting the Worldstone housed within Mount Arreat. Realizing that the Worldstone cannot be restored, Tyrael reluctantly destroys it. The resulting explosion destroys Mount Arreat, ravages much of the countryside, and decimates Baal's army.