Actually - as pointed out in WarCraft III: The Frozen Throne - Illidan is under the control of the demon lord Kil'Jaeden, in WarCraft III Illidan was sent to destroy the Lich King using the Eye of Sargeras, but Malfurion's brother stopped him in belief that Illidan was destroying the complete Northrend, the roof of the world as they call it, so that plan failed. As far as I can see Kil'Jaeden still has the control over Illidan since the demon knows the location of Illidan's stronghold - which once belonged to the pitlord Mannoroth, the true lord of Outland - the Black Temple as they call it in World of WarCraft.
Quoting final Blood Elf (Human) campaign after defeating Mannoroth and taking control over the Chaos/Fel Orcs:
Prince Kael'thas: What is this, Vashj? Where did this storm come from?
Lady Vashj: Keep your head down, fool! Something terrible is drawing near!
Kil'jaeden: Foolish little mongrel. You failed to destroy the Frozen Throne as I commanded. And still you thought to hide from me in this forsaken back water! I thought you to be more cunning, Illidan.
Illidan Stormrage: Kil'jaeden! I was merely set back. I was attempting to bolster my forces. The Lich King will be destroyed. I promise you!
Kil'jaeden: Indeed? Still, these servitors you've gathered show some promise. I will give you one last chance, Illidan. Destroy the Frozen Throne, or face my eternal wrath!
Lady Vashj: The naga are yours to command, Lord Illidan. Where you go, we follow.
Prince Kael'thas: The blood elves are yours, as well, master. We will drive the Scourge before us -- and shatter the Frozen Throne as you command.
Birdin, Human Holy Paladin 61 - EU Stormrage
The Black Citadel
Leayani, Blood Elf Frost Mage 15 - EU Stormrage
Legacy of the Highborne