Diablo 3 Story: Almost 20 years have passed since Diablo has been cast away from the World of Sanctuary by a group of brave heroes. The Lord of Terror and two of his brothers, Mephisto and Baal were slain but many mortals have sacrificed their lives to save their homeland from the burning fires of Hell. The Archangel Tyrael was forced to break the sacred laws of the High Heavens and interfere with the mortal realm, destroying the World Stone beneath Mount Arreat. The sacred mountain was destroyed in the process, leaving the mighty barbarians without their homeland.
Before he was slain, Diablo managed to corrupt the powerful witch named Adria. Through his power he managed to conceive Leah, daughter of Adria. The little girl was abandoned by her mother but Deckard Cain, last living member of the other of the Horadrim took the child and raised her as his own.