The Sin War is a tome found on the 8th level of the labyrinth in the original Diablo. It tells the tale of the ensuing conflict of the Sin War.
It reads as follows:
"Many Demons traveled to the mortal realm in search of the Three Brothers. These Demons were followed to the mortal plane by Angels who hunted them throughout the vast cities of the East. The Angels allied themselves with a secretive order of mortal Magi named the Horadrim, who quickly became adept at hunting Demons. They also made many dark enemies in the underworlds."