Great Conflict
From Diablo Wiki
The Great Conflict, occasionally alluded to in the quest and flavor text in Diablo II, seems to be the "official" name for the struggle between the forces of the Light and those of Darkness. The rules of engagement, if any, are never discussed but can be inferred to exist as only Tyrael directly intervenes on the mortal plane against the machinations of Hell while the other angels (such as Hadriel) refrain from action; Tyrael is viewed as a "rebel" by the Heavens due to his actions in this regard, according to Deckard Cain's dialogue at the beginning of Act IV. The ultimate aim of the Conflict is also never discussed, but it appears that the goal of Hell is to overthrow, occupy or destroy Heaven, while Heaven's aim seems to be the maintenance of the status quo. How exactly this is to be accomplished and the role the mortal world plays in the Conflict is left to the imagination.
Quote from the book
On Level 4 in Diablo I, a tome titled "The Great Conflict," surrounded by sconces, will voice you the following as soon as you open it:
"Take heed and bear witness to the truths that lie herein, for they are the last legacy of the Horadrim. There is a war that rages on even now, beyond the fields that we know--between the utopian kingdoms of the High Heavens and the chaotic pits of the Burning Hells. This war is known as the Great Conflict, and it has raged and burned longer than any of the stars in the sky. Neither side ever gains sway for long as the forces of light and darkness constantly vie for control over all creation."
