For the Great Evil, see Diablo.
Diablo, the Lord of Terror is a tome that can be found in Act II of Diablo III, in one of Cain's Lost Satchels.
It reads as follows:
Diablo was the youngest of the Prime Evils, but I found him the most dangerous, for his power over terror left him incapable of feeling fear. Though he had easily possessed and corrupted many humans, Diablo's essence was finally trapped in a soulstone and banished to the unfathomable Abyss twenty years ago. - Deckard Cain