Joramun

Joramun is an unseen character in Game of Thrones, appearing only in the only appears in "The Complete Guide to Westeros: The History of the Night's Watch and the Night's King" featurette. He was a legendary King-Beyond-the-Wall.

According to legend, Joramun gathered the Free Folk to battle the Night's King, who had made the Night's Watch into his personal army. He joined forces with the Stark King of Winter and together they brought down the Night's King.

In the books
In the A Song of Ice and Fire novels, Joramun is a legendary King-Beyond-the-Wall who not only fought the Night's King, but also blew the Horn of Winter to wake giants from the earth.