| Illidan, twin brother of Malfurion, practiced the Highborne magic. In his youth, he attempted to master the druidic forces, as his brother had, but the sorcery called to him in a way that the magic of the land could not. Though he was not a Highborne himself, he became the personal caster of the military leader Ravencrest.
When Archimonde's invasion of Azeroth had begun and Azshara's treachery became known, Malfurion convinced Illidan to leave his queen and foresake the Highborne magic. Illidan followed his brother. But as Cenarius and the Dragons entered the battle, Malfurion came to understand that their adversaries were too powerful to fall in combat. To end the invasion, he brazenly plotted the destruction of the Well of Eternity.
The idea appalled Illidan. The Well was the source of his magic and its loss was a price far too dear for him to pay, no matter the consequences. The Satyr Xavius seized upon his doubts, exploiting his confusion to plant seeds of distrust and hatefulness into Illidan's mind. Xavius's actions were shrewd and magical and Illidan fell under his sway. The Night Elf found that he increasingly admired the powers of the Burning Legion, seeing a magical purity that underlay their chaotic behavior. Where the Night Elves struggled to maintain their ground, the Burning Legion seemed to grow ever stronger. The sight of Tyrande Whisperwind, the secret object of his desire, in the arms of his brother Malfurion shattered his final link with his people.
In secret he stole back to Azshara's palace. There he warned her of the coming conflict, betraying his brother in the hope of sparing the Well of Eternity. Upon learning of Malfurion's plan, the queen prepared for battle. When the joint forces of Tyrande and Malfurion reached her palace, Azshara and her followers unleashed catastrophic magics upon them, slaughtering thousands. During the battle, Illidan slipped away, fearing not for himself but for the Well of Eternity.
While his brother and his queen fought in battle, Illidan knelt beside the Well of Eternity. Convinced of the superiority of the Burning Legion, he fully expected the Night Elves to fall in battle. In order that he might maintain some power in a world ruled by Demons, Illidan filled a set of seven vials with the waters of the Well. His work complete, he vanished into the shadows.
After the Sundering, Illidan scaled the peaks of Mount Hyjal, where he found a small, tranquil lake. There he poured the contents of one of the vials into the waters. The chaotic energies quickly manifested, tainting the lake and twisting it into a new Well of Eternity. Illidan's joy was short-lived however, when Malfurion and Tyrande tracked and apprehended him.
Unable to accept that his brother had committed such treachery, Malfurion tried again to explain to Illidan the folly of his ways. The magic, he insisted, was chaotic by nature, and it could only bring about destruction so long as it existed. Illidan refused to listen, so enraptured by the magic's power that his brother seemed to him an unknowing fool. His preservation of the Well, he thought, vindicated his treachery. He had saved the power. Future generations would know magic. Sorcery would not perish.
The lack of remorse shook Malfurion to his core and he raged at his brother, understanding now that Illidan was lost forever to the magic's sway. He ordered him imprisoned deep below Hyjal in a jail kept far from sight and mind. To Tyrande he confessed his greatest fear: If Illidan were ever freed from his prison, his use of the magic would lure the demons back to Azeroth.
This is your answer and sorry for so long post.
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You're not a hero -- you're just a walking corpse. | 
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