The Night King formerly known as Artel Morteus is the master and the first of the White Walkers, having existed since the age of the First Men. He is also the supreme leader of the Army of the Dead.
Prior to the Hundred Year Darkness he was a Grey Jedi necromancer sought out by the Sith Lord Sorzeus Syn to reanimate corpses in the newly formed Sith Orders war against the Jedi. Upon his refusal he was exiled to Tellure Terra and captured by the Children of the Forest, and turned into the first White Walker, his necromancy allowing him to turn other humans into White Walkers and raise the dead as wights. His ultimate goal is to wipe out all humanity on Tellure Terra and establish an eternal winter.