"Twenty years ago, Baal was loosed upon the world. He besieged the barbarian capital of Sescheron and used treachery to access the Worldstone Chamber, circumventing the three ancient guardians Talic, Madawc, and Korlic. In the end, he succeeded in corrupting the great artifact. Though Baal was slain by heroes, his damage was irreversible."

~Deckard Cain

A World lore book can be found on the Misplaced Pack, among The Keep Depths (Diablo III Act III). Part of the Achievement Historian of Arreat.