During the invasion of the Grey Valley by the Burning Legion, Illidan absorbed the energy of a powerful Warlock artifact called the 'Skull of Gul'dan' and transformed into a demon-like creature. In the process, he acquired incredible strength and part of Gul'dan's memories, particularly those concerning the Tomb of Sargeras.
He then met Kil'Jaeden, the demon responsible for what had happened. The Lich King Ner'zhul had become too powerful and was now too difficult to control. Kil'Jaeden ordered Illidan to eliminate Ner'zhul. As part of the deal, Illidan would receive more power, and be granted a place as a leader within the Burning Legion.
The corrupted Illidan immediately agreed and began his preparations to destroy the frozen throne that imprisoned the Lich King's soul. He needed a powerful artifact, so he summoned the Naga from the Dark Deep Sea Lair to find the island where the legendary tomb of Sargeras was.
