Arthas Menethil was the prince of Lordaeron around the start of the Third War. However, as the Scourge tore the Kingdom of Lordaeron apart, Arthas chased them until he acquired the cursed blade of Frostmourne. Lich King Ner’zhul used the blade as a conduit to manipulate Arthas and eventually turn him into a mad servant of the Scourge.