The Frozen Throne, with the Lich King seated upon it.
The Frozen Throne is the pinnacle of a frozen spire and the receptacle of the soul of the former shaman, Ner'zhul. From the Frozen Throne, Ner'zhul directed the formation of the Scourge as it rampaged across Lordaeron. The runeblade Frostmourne was sealed within the icy prison which contained Ner'zhul, but the former shaman thrust the blade from his prison, that it would find his chosen champion.
The frozen spire, where the Frozen Throne is located at the top.
After the Third War, the fallen prince Arthas Menethil arrived in Northrend with the goal of protecting the Throne, after hearing from Ner'zhul that Illidan's Forces had come with the intent of destroying Ner'zhul. Illidan's Forces were defeated by the Scourge as Arthas ascended the spire, all the way to the Frozen Throne. Arthas "released" Ner'zhul from his prison, and the Crown of Domination fell to his feet. Arthas placed the Crown upon his head, merging the souls of Arthas and Ner'zhul to become the Lich King. Arthas sat upon the Frozen Throne and slept for years, as the Scourge constructed the famous Icecrown Citadel around the spire.
During the War against the Lich King, a connection was revealed between the Lich King and the Frozen Throne; the Lich King could only be slain on the Throne, where Arthas merged with Ner'zhul to become the entity. Eventually, Tirion Fordring led the Ashen Verdict against the Citadel, and arrived at the Throne, with Azerothian Heroes following him. After a deadly battle that almost resulted in the doom of the world, the Lich King finally fell; however, the Lich King had become a necessity, in order to control the Scourge. Tirion contemplated taking up the role until Bolvar Fordragon, alive even after the torture of the Lich King, volunteered to take up the role. The Crown of Domination was placed on Bolvar's head, and, to this day, Bolvar sits upon the Frozen Throne.