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.