Izual is a fallen angel in demon form. Distrusted by the High Heavens and Burning hells alike.
Background
Izual was once a lieutenant in the armies of the High Heavens and the bearer of the legendary angelic runeblade, Azurewrath, forged by the Archangel Tyrael himself. The two were also good friends, and he served as Tyrael's trusted right hand.
Izual fell during his assault on the Hellforge as the Shadowfang blade was nearing completion, and his spirit was corrupted by the three Prime Evils, Diablo, Baal and Mephisto. He eventually revealed how Soulstones can be corrupted and turned to suit the three brothers' aims. He also revealed all his knowledge of the Hight Heavens, becoming an important but untrustworthy asset of the Burning Hells.
Izual has been rejected by both sides; the Heavens hated him for his corruption, and Hell also distrusted him for once being an angel.
His soul was encased within a hideous, demonic body of immense strength, dwelling in the burned out plains of Hell. This form was eventually killed by the Hero of Diablo II, revealing the extent to which he was corrupted by evil and why soulstones failed to work. By killing the body, Izual's soul was freed.