Diablo: The Sin War Book One: Birthright (Blizzard Legends) - Richard A. Knaak

Since the beginning of time, the angelic forces of the High Heavens and the demonic hordes of the Burning Hells have been locked in eternal conflict for the fate of all Creation. That struggle has now spilled over into Sanctuary -- the world of men. Determined to win mankind over to their respective causes, the forces of good and evil wage a secret war for mortal souls. This is the tale of the Sin War -- the conflict that would forever change the destiny of man.