Ner'zhul, the Lich King, knew that his time was short. Imprisoned within the Frozen Throne, he suspected that Kil'jaeden would send his agents to destroy him. The damage caused by Illidan's spell had ruptured the Frozen Throne; thus, the Lich King was losing his power daily. Desperate to save himself, he called his greatest mortal servant to his side: the death knight Prince Arthas.
Though his powers were drained by the Lich King's weakness, Arthas had been involved in a civil war in Lordaeron. Half of the standing undead forces, led by the banshee Sylvanas Windrunner, staged a coup for control over the undead empire. Arthas, called by the Lich King, was forced to leave the Scourge in the hands of his lieutenant, Kel'Thuzad, as the war escalated throughout the Plaguelands.
Ultimately, Sylvanas and her rebel undead (known as the Forsaken) claimed the ruined capital city of Lordaeron as their own. Constructing their own bastion beneath the wrecked city, the Forsaken vowed to defeat the Scourge and drive Kel'Thuzad and his minions from the land.
Weakened, but determined to save his master, Arthas reached Northrend only to find Illidan's naga and blood elves waiting for him. He and his nerubian allies raced against Illidan's forces to reach the Icecrown Glacier and defend the Frozen Throne.