Who: Illidan Stormrage, the Betrayer.
What: Illidan is a Night Elf by birth, but after consuming a demonic skull, plus a bunch of other dirty deals with demons, he can now be considered a Night Elf/demon hybrid. Amusingly, he is a demon hunter by profession.
History: Illidan is the twin brother of Malfurion Stormrage, the first Night Elf druid. Illidan and Malfurion competed for the love of Tyrande Whisperwind, a Night Elf priestess and the current boss of Darnassus. When Archimonde's Burning Legion first invaded Azeroth, Malfurion and Illidan worked to stop him, but Illidan's jealousy over Tyrande eventually led him to betray his brother. Malfurion planned to destroy the Well of Eternity, the fount of arcane magic on Azeroth. Illidan spirited away a couple vials of the well's water before its destruction so he could still have some power after the Burning Legion took over. Illidan used the vials to create a new Well of Eternity, but was caught by his brother and sentenced to be forever imprisoned beneath the earth.
Ten thousand years after Illidan was imprisoned, Tyrande freed him so he could help fight the Scourge. Malfurion and Illidan's keeper, Maiev Shadowsong, opposed this decision and vowed to track down Illidan. Meanwhile, Illidan led some night elves to Felwood and briefly fought Arthas, then the champion of the Lich King. Arthas told him that destroying an artifact called the Skull of Gul'Dan would seriously weaken the Burning Legion. When Illidan found the Skull, he consumed it instead of destroying it, turning himself into a demon hybrid. Illidan thought this would help defeat the Burning Legion and make him look better to Tyrande and Malfurion, but instead they banished him after he defeated the forces of Tichondrius.
Illidan was then visited by Kil'Jaeden, the commander of the Burning Legion. Kil'Jaeden commanded him to destroy the Lich King, Ner'Zhul, since Ner'Zhul had grown too powerful for the Legion to control. Illidan gathered an army of Naga and set forth to find the powerful Eye of Sargeras (killing a large number of his former captors in the process.) He took the Eye to Lordaeron and tried to melt the ice of the polar ice cap and destroy Ner'Zhul's land. Yay for global warming as a plot device! But Malfurion and Maiev stopped him, believing his plan would destroy the whole world.
After Illidan saved Tyrande from a horde of undead, Malfurion allowed him to escape to Outland to get away from Kil'Jaeden's wrath. Maiev followed and recaptured him, but the blood elf Kael'Thas and the naga Lady Vashj freed him. Together, Illidan, Vashj and Kael'Thas conquered Outland. Things were looking good for Illidan -- he had some loyal lieutenants, a new palace, and a whole world to himself. Unfortunately, Outland was metaphorically in Kil'Jaeden's backyard, and the demon soon tracked down Illidan and made him once again promise to destroy the Lich King.
Illidan and his forces returned to Azeroth and fought up to the Lich King's throne. But Ner'Zhul had called his champion, Arthas, to the throne to receive his powers. Arthas and Illidan met in battle once more, and Arthas defeated Illidan and left him for dead. Arthas ascended the Frozen Throne to become the new Lich King, and Illidan was dragged back to Outland in disgrace.
Where he is now: Illidan reigns over Outland from the Black Temple, along with his lieutenants Kael'Thas and Lady Vashj. He controls his own faction, the Illidari, but it is unknown whether he is serving the Burning Legion or his own interests.
Identifying characteristics: According to the official World of Warcraft site, Illidan has retained most of his night elf characteristics despite becoming part-demon, and is therefore pretty unpredictable. Illidan has had his eyes gouged out -- he now has magical vision bestowed by the Burning Legion -- and he has bat-like wings and horns in his demon form.
Why we have to kill him: If Illidan's serving the Burning Legion, he's helping them plan the destruction of Azeroth. If he's not serving the Burning Legion, then we're basically invading his country for revenge, which makes us kind of jerks. Most likely, Illidan has some sort of plot against Azeroth that requires us to step in and Save The World Again. Plus, there's a chance we won't kill him -- rumor is that we merely "get in his way."
You: Are not prepared.
For more information: Illidan on the World of Warcraft site, Illidan on WoWWiki, Arthas v. lllidan on Youtube.
Know Your Lore is not infallible and is still working through the Warcraft games, so most of the information comes from the official World of Warcraft site and WoWWiki. Feel free to submit possible corrections -- and recommendations for other Warcraft figures to cover -- in your comments!