Illidan, by contrast, was somewhat different. Although he also absorbed demonic power, he slowly extracted and refined that Fel energy, integrating it into power he could command. Illidan's body had been completely altered by Fel, but his soul still belonged entirely to him. He had never sold his soul to demons.
Although demons could grant others power, these most violent of plunderers always stole your most precious treasure.
Finally, Kael'thas revealed his predicament. "I can no longer break away from Kil'jaeden. Once I attempt to lead my troops back to Azeroth aboard the Tempest Keep warship, those demons lurking in the shadows will undoubtedly attack us directly. Kil'jaeden's most loyal minions-Doom Lord Kazzak, the Doomwalker Burning Legion, the Void Lord, Conqueror Razelikh, and even Illidan-will all obey his orders and attack us. Besides, in this vast universe, we have no star charts. We don't know where Azeroth is. Without the protection of the Ley Line System, what good would fleeing to another planet do? Kil'jaeden would still capture us."
"Returning to Azeroth through the Dark Portal is even more impossible. Doom Lord Kazzak has been watching it like a hawk, and the Sha'tar allied forces in Shattrath and the others won't let us retreat safely either." Kael'thas spoke with some self-mockery. "Back then, I only decided to attack the Draenei after learning that this world had Eredar Wizards who used the Holy Light. I have to say, these Naaru ships have been a great help. The Holy Light permeating this place suppresses Fel and demons extremely well. Only here can I suppress the corruption of Fel and slow Kil'jaeden's influence over me."
There were many Eredar in the Burning Legion, but most of them were demons. The Elves had been dealing with the Eredar since the War of the Ancients.