Izual's Descent is a tome found in Act IV of Diablo III, found in The Great Span.
It reads as follows:
The angel Izual was once Tyrael's lieutenant, but he was captured by the enemy after an ill-advised assault upon the Hellforge. Izual surrendered to demonic corruption and revealed the secrets of the soulstones to the Prime Evils. Tyrael claimed that Izual was slain years ago, but his replacement has never emerged from the Crystal Arch... - Archangel Itherael