Arthas Menethil

The Lich King is the master and lord of the Scourge, which he rules telepathically from the Frozen Throne atop the Icecrown Glacier.

Background
Arthas Menethil, Crown Prince of Lordaeron and Knight of the Silver Hand, was the son of King Terenas Menethil II and heir to the throne. He was trained as a paladin by Uther the Lightbringer, and had a romantic relationship with the kind sorceress Jaina Proudmoore.

Despite his promising beginnings, Arthas became one of the most powerful and evil beings Azeroth would ever know. Taking up the cursed runeblade Frostmourne, he became a death knight, led the Scourge in destroying Lordaeron, and merged with the Lich King. Ruling as the dominant personality of the Lich King for years afterwards.