Illidan Stormrage is rightly called the Betrayer, as throughout the millennia of his life he has deceived friends and family alike for power.
The night elf Illidan Stormrage was born before the War of the Ancients, a conflict that erupted over 10,000 years prior to the Third War, when the chaotic Burning Legion reinvaded Azeroth. Driven by his thirst for power (and his desire to impress Tyrande Whisperwind), Illidan made a pact with the Legion to secure the entrance of its leader, Sargeras, into Azeroth through the might of an artifact called the Demon Soul. The efforts of Malfurion Stormrage, Illidan's twin brother, disrupted the Legion's plot; the demons were defeated, and Illidan was imprisoned.
During the Third War, Tyrande released Illidan from millennia of imprisonment, hoping that the Betrayer would redeem himself by battling a returned Burning Legion. Though Illidan fought to defend his people, he soon slipped into darkness: after absorbing the energies of the demonic Skull of Gul'dan, Illidan became a demon, an act for which he was banished by Malfurion. Fleeing the wrath of the night elves, the twice-condemned Illidan allied himself once again with the only entity that would accept him--the Legion. Illidan's demonic masters sent him to destroy the Lich King, who had broken free of their influence, but Illidan failed to do so. To protect himself from the Legion's vengeance, Illidan hid on Outland, a destroyed world that he would eventually seek to rule.