Escape to Outland

After Malfurion let him go, Illidan created a portal to Outland and fled immediately, pursued by Maiev. Now that he had failed to destroy Ner'zhul, he knew that Kil'jaeden's wrath would not spare him, so he wanted to find a world where he could remain unmolested. He felt that Outland, the ruined remains of Draenor, was just such a place. 

Illidan was chased down on the shattered world until he was captured by Maiev and the Watchers, and imprisoned once again. But he was saved by Kael and Vashj. Illidan accepted the allegiance of the blood elves, and made Kael his second-in-command. The Sin'dorei, coupled with the naga, would be very invaluable to his plans. Illidan continued his original plan - to rid Outland of demonic influence so that he could stay out of Kil'jaeden's grasp. To do this, they laid siege to the Black Temple of Magtheridon, the Pit Lord who had taken control of the world. But first, Illidan systematically shut down his dimensional gates to stem the flow of reinforcements. Eventually, they succeeded. 

When they came to the Black Temple, Illidan was approached by Akama of the Broken, who pledged the allegiance of his race. Akama's broken felt indebted to Illidan and his army for aiding them in their fight against the fel orcs of Magtheridon, who had laid siege to their village with the intent of slaughtering them all. They laid siege to the Black Temple and destroyed Magtheridon's defenses, and then defeated the Pit Lord in battle himself. Magtheridon noted wryly that Illidan had great power, and asked if the Legion had sent him as a test. Illidan laughed, saying that he was not a test, but a replacement, and defeated Magtheridon. It has recently been revealed that Magtheridon is still alive, imprisoned by Illidan beneath Hellfire Citadel and kept sealed in place by the fel orc jailor Keli'dan the Breaker.