At this time, the undead Scourge had essentially transformed Lordaeron and Quel'Thalas into the toxic Plaguelands. There were only a few pockets of Alliance resistance forces left. One such group, consisting primarily of high elves, was led by the last of the Sunstrider dynasty: Prince Kael'thas.
Kael, an accomplished wizard himself, grew wary of the failing Alliance. The high elves grieved for the loss of their homeland and decided to call themselves blood elves in honor of their fallen people. Yet as they worked to keep the Scourge at bay, they suffered greatly at being cut off from the Sunwell that had empowered them.
Desperate to find a cure for his people's racial addiction to magic, Kael did the unthinkable: he embraced his people's Highborne ancestry and joined with Illidan and his naga in hopes of finding a new magical power source upon which to feed. The remaining Alliance commanders condemned the blood elves as traitors and cast them out for good.
With no place left to go, Kael and his blood elves followed Lady Vashj to Outland to help contest the warden, Maiev, who had recaptured Illidan. With the combined naga and blood elf forces, they managed to defeat Maiev and free Illidan from her grasp. Based in Outland, Illidan gathered his forces for a second strike against the Lich King and his fortress of Icecrown.