It's Bolvar Fordragon, Paladin of the Silver Hand, Commander of the Alliance's combined military forces in Northrend, former Regent of Stormwind, and fallen hero of the Wrathgate. He was thought dead at the Wrathgate, but had in fact survived and been captured by the Scourge, and tortured, with the eventual intention of being made into a Death Knight, much like Saurfang the Younger. However, unlike Saurfang, Bolvar resisted until Arthas's defeat.
After Arthas Menethil, mortal host of the Lich King, was slain, Tirion Fordring took up the Helm of Domination, the crown of the Lich King, preparing to don it himself. As he did so, Bolvar intervened, calling from his seat upon the Frozen Throne, his body warped by the red dragonflight's fire. As a last act of service to Azeroth, Bolvar asked that Tirion place the Helm of Domination on his head. As the self-appointed "Jailer of the Damned," Bolvar seeks to keep the Scourge under control. As Bolvar encased himself and the Frozen Throne once again in ice, he warned Tirion that no one could know what happened. Tirion would have to tell the world that the Lich King was dead, yet as his voice changed, he added that Bolvar Fordragon had died with him.