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.

The Lich King Triumphant

Even weakened as he was, Arthas ultimately outmaneuvered Illidan and reached the Frozen Throne first. Using his runeblade, Frostmourne, Arthas shattered the Lich King's icy prison and thereby released Ner'zhul's enchanted helm and breastplate. Arthas placed the unimaginably powerful helm on his head and became the new Lich King. Ner'zhul and Arthas' spirits fused into a single mighty being, just as Ner'zhul had always planned. Illidan and his troops were forced to flee back to Outland in disgrace, while Arthas became one of the most powerful entities the world had ever known.

Currently Arthas, the new and immortal Lich King, resides in Northrend; he is rumored to be rebuilding the citadel of Icecrown. His trusted lieutenant, Kel'Thuzad, commands the Scourge in the Plaguelands. Sylvanas and her rebel Forsaken hold only the Tirisfal Glades, a small portion of the war-torn kingdom.