Night King

The Night's King 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 is a legendary figure known both in the Seven Kingdoms and among the Free Folk dwelling Beyond the Wall.

According to legend, the Night's King was originally a Lord Commander of the Night's Watch who found in the Haunted Forest a cold woman with bright blue eyes, seemingly a female White Walker. He took her to the other side of the Wall and declared himself "Night's King". For thirteen years the two ruled over the brothers of the Night's Watch, performing human sacrifices. The Free Folk rallied under the banner of a King-Beyond-the-Wall and marched against the Nightfort, which the Night's King had taken as his seat, defeating him with the aid of House Stark.

In the books
In the A Song of Ice and Fire novels, the Night's King was the thirteenth Lord Commander of the Night's Watch and a fearless warrior. However he fell in love with a woman "with skin as white as the moon and eyes like blue stars"; he loved her though "her skin was cold as ice", and when he gave his seed to her he gave her his soul as well. He brought her to the Nightfort and bound the brothers of the Night's Watch to his will through sorcery. The King in the North and Joramun, the King-Beyond-the-Wall, joined forces to defeat him. After he was killed, it was discovered that he had been sacrificing to the Others - the White Walkers - and all records of him were destroyed, while pronouncing his name was forbidden. Nevertheless, it is believed he may have been a Bolton, a Magnar of Skagos, an Umber, a Flint, a Norrey, or a Woodfoot, though it is possible he may have actually been a Stark, brother to the King-in-the-North, named Bran.