"Betrayer... In truth, it was I who was betrayed. Still, I am hunted. Still, I am hated. Now, my blind eyes see what others cannot. And sometimes the hand of fate must be forced! Now go forth... unleash the tides of Doom... Upon all those... Who would oppose us."
from Warcraft 3 cinematic
Illidan Stormrage was the self-proclaimed Lord of Outland, ruling from the Black Temple. He was born a night elf and, as stated by Maiev Shadowsong, became "neither night elf nor demon, but something more". He was the twin brother of Malfurion Stormrage and was in love with Tyrande Whisperwind. Once an unusually gifted sorcerer, the extent of his powers became difficult to classify due to his powers increasing in large bursts as a Demon Hunter and his having absorbed the powers of the dangerous arcane object known as the Skull of Gul'dan.
His pursuit of power and arcane mastery led him to commit a number of horrific acts against his own people and the races of Azeroth, including defecting to Sargeras during the War of the Ancients and creating the second Well of Eternity. For his actions, he was imprisoned for ten thousand years, until his release during the Third War. He came to be called the Betrayer for his acts against the night elf people and carried the title of Lord of Outland.
Illidan copyright Blizzard:Warcraft
Art by Jazz Siy
Illidan: King or Pawn?
In the last battle of the War of the Ancients, Illidan begins casting a spell to reverse Sargeras's portal. Tyrande senses that several someones have augmented his spell — and these someones were darker than even Sargeras. These beings were clearly the Old Gods, who wished to redirect Sargeras' portal to their prison beyond the mortal plane instead.
It is also quite possible that Illidan's more recent actions are also aided by the Old Gods. Given that the naga are presumed to follow them, it has been suggested that Vashj and her ilk answered Illidan's summons as part of the new plan (though given Azshara's previous association with him, there are other explanations). Illidan might be an attempt to position a powerful pawn in the midst of a strategic location.
P.S. This is only a fanart.
Illidan Background taken from http://www.wowwiki.com/Illidan_Stormrage