Kanrethad was present, ostensibly as a member of the Alliance, when Illidan Stormrage was killed during the siege of the Black Temple.
Following the battle he shared the secrets of [Metamorphosis] with the council, who then spread it to other warlocks. After Deathwing's downfall, he called the council together to share the secrets he had discovered behind Illidan's transformation, and sent them out gather the powers of Ragnaros and Cho'gall, and return in a year's time. He partnered himself with the undead warlock Jubeka Shadowbreaker and travelled through the Dark Portal, to the fel-saturated realm of Outland.[1]
Travels Through Outland
Kanrethad and Jubeka travelled throughout Outland, studying the various demons of the fel-infested land - the fel imps, voidlords, observers, and doomguards - seeking to learn the sources of their power, as well as the secret to how Illidan was able to control them while not being a servant of the Burning Legion. From a captured doomguard in the Blade's Edge Mountains, Kanrethad learned that the doomguards had been servants of the Titans who punished those who worked in sacrificial magics. Near the Hand of Gul'dan, the great fel volcano in the center of Shadowmoon Valley, Kanrethad revealed his ultimate intention - to possibly become a demonic entity himself, recognizing that it came with incredible risk. He recognized that Jubeka feared that he would do to Azeroth what Gul'dan had done to Draenor, the world Outland had once been; he revealed that he had chosen Jubeka as his companion because if he failed, she would have to banish him forever.
Apotheosis and Banishment
Kanrethad and Jubeka returned to the Black Temple, evading the Ashtongue Deathsworn under Akama who had reclaimed the temple after Illidan's death. Entering the Shrine of Lost Souls, Kanrethad recognized it as the power source that allowed Illidan to control demons without their being corrupted by the fel energies endemic to the Legion, and theorized that it had been originally intended as the power source to replace the Sunwell when Kael'thas Sunstrider and his blood elves aligned with him. Since Illidan's death, it grew in strength and power, waiting to be harnessed. The two warlocks prepared a crystal to absorb the arcane magics within the Shrine, leaving it to do its work when they heard Ashtongue patrols hunting them.
An adventurer who studied warlock magics soon came on their trail, locating a strange book known as [The Codex of Xerrath] from a creature on the Isle of Thunder in Pandaria. Upon speaking to their warlock trainer, they learned of the Council and its mission after Deathwing's demise, and followed the trail left by Jubeka into the Black Temple. Reclaiming the now-empowered crystal and taking it to the summit, the warlock witnessed the opening of a massive gateway, and from it stepped Kanrethad in a monstrous demonic form, utterly corrupted by absorbing too much fel energy. He declared that the Black Harvest would enslave hordes of demons and destroy anyone who stood against them, on Outland or on Azeroth. The warlock who stood against him used all of the powers at their disposal, taking control of Kanrethad's pit lord minion and using its powers to augment their own. Though Kanrethad's powers were formidable, the warlock perservered long enough for Jubeka to arrive and fulfill Kanrethad's earlier request to banish him. The warlock then absorbed some of the fel magic, corrupting their fire spells into a sickly fel-green.
With Kanrethad defeated, Jubeka brought him to the Altar of Damnation, in the shadow of the Hand of Gul'dan, where she keeps him banished forever.