The March on Icecrown

As Illidan rallied the forces of Outland under a new banner, a storm of fire and smoke descended upon the Black Temple, and Kil'jaeden appeared in all his unholy glory. Chastising Illidan for his foolhardy attempts to flee his wrath, Illidan quickly claimed that he had been gathering his forces for a second attack on the Frozen Throne. Kil'jaeden, seeing that Illidan's new comrades "showed some promise", agreed to give Illidan one more chance to appease the demons. 

Illidan, Vashj, and Kael laid siege to Northrend and battled Anub'arak's forces as they trudged through the snow towards the Icecrown glacier. But Ner'zhul, knowing he would be overrun if he didn't do something, called Arthas to Northrend to complete the plan he had set in motion so many months before. Finally, Illidan's forces arrived at Icecrown as Arthas and Anub'arak dug their way out of Azjol-Nerub, and the two factions squared off in a titanic battle as they tried to gain control of the four mystical obelisks surrounding the glacier. After a long battle, and control going back and forth between the two enemies, Arthas activated all obelisks, and the doors to the Frozen Throne were opened. 

But Illidan was not finished yet. Meeting the weakened Arthas at the glacier's base, the two warriors engaged in single combat. After a few minutes of exchanging blows, however, Arthas overcame Illidan's defenses and cut him down. Illidan Stormrage fell in the snow, dead, or so it seemed. He was badly wounded, but not killed. After it became clear for Vashj and Kael that they could not destroy the Frozen Throne, they retreated, eventually back to Outland, taking Illidan with them. 

Note that the in-game animation between Illidan and Arthas was originally planned to be a video cinematic. Time constraints, however, forced the scene to be done inside the game. The developers have said that their greatest regret about this change is that many assumed Illidan died, when the video would have revealed that Illidan survived to watch Arthas's transformation.