Izual
A fallen angel, Izual was once a lieutenant in the armies of Heavens and the bearer of the legendary angelic runeblade, Azurewrath, forged by the Archangel Tyrael himself. He was also a good friend of the Archangel, and served as his trusted right hand. Izual fell during his assault on the underworld Hellforge as the Shadowfang blade was nearing completion, and his spirit was corrupted by the three prime evils, Diablo, Baal and Mephisto, who eventually were told by him of the way soulstones can be corrupted and turned to suit three brothers' aims in addition to all his knowledge of Heavens, becoming an important, but untrustworthy asset of Hell. According to Blizzard, Izual had 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 adventurer in 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.