system
Given the following post, respond as if you're commenting on Reddit:
human
Can someone explain the Diablo Story leading up to D3 (LI5)?

I was one of those people who played D1 and D2 for years but never really knew anything about the story whatsoever. I've been trying recently to read a lot about it but I just dont understand how in Diablo II we killed Diablo and his soul stone, yet hes coming back? And I dont understand the Worldstone at all :(

The goal of this is to be able to go into D3 without being clueless and knowing who is dead and who isn't :)
gpt
Originally, the 3 Lords of Hell (Mephistopheles, Diablo, and Baal) were trapped in what are called soulstones. Diablo's was buried and a monastary built over it in a town called Tristram. A king decided to turn the monastary into a cathedral. A man named Lazarus was tricked/lured into breaking Diablo's soulstone. Diablo possessed the king's son to do bad stuff. 

Now you come in as the hero. The hero is awesome and kills stuff and kills Diablo's mortal form, leaving him in a soulstone again. The hero drives the stone into his own skull to hopefully contain him. But this is what Diablo wanted, as the hero is stronger. Diablo also corrupted the other two heroes (you were the warrior for the story). The rogue became Blood Raven and the Sorcerer became the Summoner.

In Diablo 2, you follow the wake of the unnamed hero who has started to become corrupted by the soulstone. You find out the truth about the soulstones. You fight lesser evils of Andariel and Duriel. You fight Mephisto, who was released by Diablo. You find out that a fallen angel, Izual, taught the evils how to corrupt the stones. You fight Diablo.