Diablo: Tristram starts to tear itself apart as a corrupt bishop kidnaps the kings son. Multiple heroes show up to attempt to save the kingdom. It’s discovered the demon causing the trouble was the exiled supreme ruler of Hell. The warrior survives and realizes the only way to attempt to stop this from happening is to try and contain Diablo in himself. As there is no other way to eliminate the soul stone Diablo is trapped in.
Diablo II: The warrior realizes he was not strong enough as neither were his companions, all 3 succumb to Diablo’s influence and perform his bidding. Diablo starts to manipulate the formerly superior warrior. Heroes attempt to stop him as his corruption starts to spread throughout the world. Diablo is succesful in freeing his two brothers and allowing multiple minor demons to assist him (who supposedly overthrew and exiled his Diablo and his brothers from hell). Furthermore it turns out the soul stones and the exile were all manipulated by Diablo, Mephisto, and Baal! Mephisto and Diablo are slain (although you can’t permanently kill a demon) and their gateway to Sanctuary destroyed thanks to Tyrael (a head angel) revealing the secrets. Baal’s where abouts are unknown.
Diablo II LoD: Turns out Diablo and Mephisto either a.)lured the heroes away from Baal or b.) Baal has ulterior motives and abandons his brother. In the end, Baal succeeds and corrupts the world stone, an ancient monument whose powers are unknown. Tyrael is forced to destroy the worldstone. The entire world of sanctuary braces for the invasion from Hell.
Diablo III: Twenty years after the events of Diablo II. Deckard Cain and Leah are in Tristram Cathedral (the same cathedral that was the setting of Diablo) investigating ancient texts regarding an ominous prophecy. A mysterious star falling from the sky strikes the Cathedral, creating a deep crater into which Deckard Cain disappears.