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During the Legion's invasion of Ashenvale, Illidan was released from his barrow prison after ten thousand years of captivity. Though he sought to appease his comrades, he soon reverted to true form and consumed the energies of a powerful warlock artifact known as the Skull of Gul'dan. By doing so, Illidan developed demonic features and vastly magnified power. He also gained some of Gul'dan's old memories - especially those of the Tomb of Sargeras, the island dungeon rumored to hold the remains of the Dark Titan, Sargeras.
Bristling with power and free to roam the world once more, Illidan set out to find his own place in the great scheme of things. However, Kil'jaeden confronted Illidan and made him an offer he could not refuse. Kil'jaeden was angered by Archimonde's defeat at Mount Hyjal, but he had greater concerns than vengeance. Sensing that his creation, the Lich King, was growing too powerful to control, Kil'jaeden ordered Illidan to destroy Ner'zhul and put an end to the undead Scourge once and for all. In exchange, Illidan would receive untold power and a true place amongst the remaining lords of the Burning Legion.
Illidan agreed and immediately set out to destroy the Frozen Throne, the icy crystal cask in which the Lich King's spirit resided. Illidan knew that he would need a mighty artifact to destroy the Frozen Throne. Using the knowledge he had gained from Gul'dan's memories, Illidan decided to seek out the Tomb of Sargeras and claim the Dark Titan's remains. He called in some old Highborne debts and lured the serpentine naga from their dark undersea lairs. Led by the cunning witch Lady Vashj the naga helped Illidan reach the Broken Isles, where Sargeras' Tomb was rumored to be located.
As Illidan set out with the naga, Warden Maiev Shadowsong began to hunt him. Maiev had been Illidan's jailor for ten thousand years and relished the prospect of recapturing him. However, Illidan outsmarted Maiev and her Watchers and succeeded in claiming the Eye of Sargeras despite their efforts. With the powerful Eye in his possession, Illidan traveled to the former wizard-city of Dalaran. Strengthened by the city's ley power lines, Illidan used the Eye to cast a destructive spell against the Lich King's citadel of Icecrown in distant Northrend. Illidan's attack shattered the Lich King's defenses and ruptured the very roof of the world. At the final moment, Illidan's destructive spell was stopped when his brother Malfurion and the Priestess Tyrande arrived to aid Maiev.
Knowing that Kil'jaeden would not be pleased with his failure to destroy the Frozen Throne, Illidan fled to the barren dimension known as Outland: the last remnants of Draenor, the orcs' former homeworld. There he planned to evade Kil'jaeden's wrath and plan his next moves. After they succeeded in stopping Illidan, Malfurion and Tyrande returned home to Ashenvale Forest to watch over their people. Maiev, however, would not quit so easily, and followed Illidan to Outland, determined to bring him to justice.
- Official lore