The Lich King is the master and lord of the Scourge, which he rules telepathically from the Frozen Throne atop the Icecrown Glacier.
Kil'jaeden the Deceiver created the Lich King from the spirit of the orcish shaman Ner'zhul to raise an undead army to conquer Azeroth for the Burning Legion. Initially trapped within the Frozen Throne with Frostmourne, the Lich King eventually betrayed Kil'jaeden and merged with the human Arthas Menethil, although Arthas became the dominant personality of this fusion. With the destruction of Frostmourne and the death of Arthas, Bolvar Fordragon took the powers of the Lich King within himself, imprisoning the master of the Scourge within the Frozen Throne once more.
The Lich King is the titular antagonist of World of Warcraft: Wrath of the Lich King.