Illidan Stormrage was born a night elf along with his twin brother Malfurion Stormrage
. Malfurion became a master of the druidic forces, while Illidan was strangely called to the sorcerer magic. During Archimonde's invasion of Azeroth, Illidan began to become curious and drawn to the powers of the Burning Legion
. At this time Illidan had strong feelings for Tyrande Whisperwind
, he was in love, but it was clear from the beginning that she was only interested in his twin brother, Malfurion.
A satyr named Xavius, knowing this, manipulated Illidan by furthering his jealous rage of his brother eventually leading to him disbanding from the defenders of the invasion.
Illidan was then given a gift for his new allegiance to Sargeras, his eyes were replaced with fire orbs allowing him the ability to see all forms of magic, and his body was also covered with arcane tattoos. After the Well of Eternity, his source of magic, was destroyed, Illidan discovered a small lake on the peak of Mount Hyjal were he would form a new Well of Eternity. He was, however, discovered by his brother and the rest of the Kaldorei leadership, who having realized Illidan's horrific treachery tried to explain that the magic that had taken a hold of him would always bring destruction to the world.
Illidan already so engulfed by the magic thought his brother to be an unknowing fool and simply claimed it would be a necessity if the Burning Legion were to return. Malfurion, horrified and distraught, understood that his brother had become a slave to the powers of the Burning Legion. He ordered his imprisonment deep below Mount Hyjal in a jail kept far from sight.
The Legion returned to Azeroth with the Scourge ten thousand years later. Illidan was rescued from his jail by Tyrande, who wanted his help to defeat the Legion in return for his freedom. With his love for her, unchanged even after all the years imprisoned, Illidan agreed to help. His brother, Malfurion, was opposed to the idea of freeing him. Illidan, wanting to prove his brother wrong, went to Feelwood to fight the Legion as promised. He was tracked by Arthas which in turn led to his knowledge of the Skull of Gul'dan. Arthas, knowing of Illidan’s thirst for power, explained all the forces he could obtain upon finding the skull. Illidan eventually obtained the skull, and instead of destroying it to end the corruption of the forest, he took it for his own use. However, the skull's powers changed him, he was transformed into a demon and his brother sensed this. Malfurion felt his brother had traded his soul for the search for power and banished him from the forests. Illidan left the night elf lands for good.
Illidan was given a chance to serve for the Legion again by Kil'jaeden after his banishment. With the promise of more power and magic, he accepted the opportunity. He was given the task of destroying the Frozen Throne. He eventually arrived on the Broken Isles during his quest where he was able to obtain the Eye of Sargeras, he was also confronted by Maiev Shadowsong which led to a great battle. Eventually Malfurion and Tyrande came with reinforcements, Illidan escaped by sea after capturing Tyrande. She went on to explain why she shunned Illidan so long ago and chose his brother. He had become too drunk with power and had forgotten his inner strength. Malfurion held on to that strength within him even after his sudden increases in power.
Illidan had finally come to grips with these feelings and began to understand. After landing on Lordaeron, Illidan made his way to Dalaran where he intended to destroy the Frozen Throne using the Eye of Sargeras. He was interrupted by Maiev and Malfurion before he could finish the spell. Maiev had told Malfurion earlier that Tyrande had been killed and identified Illidan as the cause. The two brothers were heartbroken, but hope was restored when Prince Kael'thas explained that Tyrande was not killed but instead was “swept away by a river’s currents.” The twins went separate ways but with the same objective, to track Tyrande. Illidan eventually found her under attack by undead forces. After defeating the undead, Illidan brought Tyrande back to his brother. For this, Malfurion told him he was free to go as long as he will never threaten the night elves again. Illidan agreed.
Since he had failed in his last chance for the Legion, he knew that Kil'jaeden would seek his death. To escape his grasp, Illidan opened a portal to Outland and fled to the ruined remains of Draenor. Although he was safe from Kil'jaeden, Illidan had been tracked once again by Maiev and the Watchers. Maiev was able to capture and imprison him again. But he was rescued by Kael and Vashj, Illidan in turn accepted allegiance to the blood elves. He then continued to rid Outland of the demonic influence to stay out of Kil'jaeden's grasp, this lead to the siege on the Black Temple. With the aid of the Broken, Illidan succeeded in defeating the current lord of Outland, Magtheridon. Illidan imprisoned Magtheridon beneath Hellfire Citadel, and claimed the Black Temple for himself.
Illidan Stormrage the Betrayer is now known as a Demon Hunter and resides within the Black Temple as the self-proclaimed Lord of Outland.