Release from prison

Illidan languished for 10,000 years in a lightless prison. Califax, a Keeper of the Grove, and a contingent of night elves kept constant guard over the Betrayer. Release came at the unexpected hands of Tyrande, who slaughtered the night elf guards in the hopes of using Illidan against the Legion, which had returned to Azeroth at the hands of the Scourge. With his love for Tyrande undampened through the millennia of confinement, Illidan agreed to help. He swore to throw back the Legion and to depart from the night elves forever.

Malfurion opposed Tyrande's decision, thinking the release of Illidan a catastrophic mistake. Frustrated that his brother remained unchanged and eager to prove to him that the demons had no hold over him, Illidan left Malfurion behind, leading a force of night elves into Felwood to hunt down the Legion. While in Felwood, he encountered Arthas, champion of the Lich King, and they engaged in combat. The fighters were equally matched, and the combatants reached an impasse. Illidan stopped the duel and demanded to know why Arthas had tracked him. Arthas responded openly, telling Illidan about the Skull of Gul'dan, the demonic artifact that had corrupted Felwood. He explained that if the artifact were destroyed, the corruption of the forests would halt. To ensure that Illidan would take the bait, Arthas spared no detail of the Skull's power, adding that his master knew of Illidan's thirst for power. Although Illidan did not trust Arthas, he nevertheless sought the Skull and its power out. 

A great Demon Gate defended the Skull as Illidan and his forces had to fight mightily to gain access to the artifact. Driven by necessity and influenced by the belief that with the increased power, he could at last redeem himself in the eyes of Tyrande, he shattered the demonic seal and used the Skull's powers for his own. Strength he found, but peril greater than power came to him at once. The battle for control of the Skull came with a heavy price and the artifact's powers changed him. Transformed into a demon, Illidan wrapped himself in shadow and utterly destroyed Tichondrius and his forces. But with victory came devastation. Tyrande and Malfurion sensed the demonic power lurking within Illidan and turned away with disgust and despair. Malfurion chastised his brother, convinced that Illidan had traded his soul for more power. Enraged, Malfurion banished his brother from the forests. 

Feeling his sacrifice and effort were unappreciated, Illidan muttered "So be it...brother", and left the night elf lands.