Game of Thrones fans are divided on whether or not the identity of the Night King is even a mystery that’s waiting to be solved. Some are convinced the chilly antagonist has a secret past waiting to be unveiled, and there are plenty of Night King fan theories out there to back that up. Others think that Night King legends are nothing more than world-building for the A Song of Ice and Fire book series, and that the character in the show is just a nameless, heartless zombie-making machine.
There are merits to both schools of thought, but this is the world of George R. R. Martin, and he loves his dark reveals and plot twists. Of all the fan theories regarding the identity of the Night King, or the Night’s King as he’s known in the books, the most convincing all revolve around the leader of the White Walkers beginning life as a Stark. And you have to admit, a direct connection between the series’ main protagonists, including Bran, and its ultimate villain would be a very George R. R. Martin-esque twist.
The Night King appears to be the ultimate antagonist in Game of Thrones, an enemy more dangerous than Cersei Lannister, Euron Greyjoy, and Ramsay Bolton combined. Although little is known about the Night King, some basic information can be gleaned from his limited scenes. We know he's old, having been turned from human to White Walker by the Children of the Forest thousands of years ago, and he's now clearly the leader of the White Walkers.
The Night King has the ability to transform human children into White Walkers, but it is unknown if this skill is unique to him, or shared by all White Walkers. He can also to raise dead humans and make them his servants, transforming them into an army of zombie-like wights. The Night King is also a master of the "come at me, bro" stare.
The common denominator between the show and book versions of the Night King is the connection between Stark and White Walker. The symbolic connections are clear: the Starks are known as the Kings of Winter, and the Night King wears an icy crown. There also seems to be some sort of connection between the Night King and both Bran Stark and Jon Snow, given his habit of staring them down.
Reddit user theDarkLordOfMordor thinks the connection, in both book and show, is quite direct. The books are filled with history of the Others, including information that their ranks are filled with Stark relatives. The Night’s King was said to be a Stark and to have sired children with an Other. There’s a strong possibility that Stark children have been turned into White Walkers, just like Craster’s were.
The story of the Night’s King is quite different in A Song of Ice and Fire, and not just because of the added apostrophe and letter. Bran recalls his education on the legendary character, who was said to be the 13th Lord Commander of the Night’s Watch. The Night's King was seduced by what sounds like a female Other, something yet to be seen in the series. She is described as having pale skin and blue eyes, which sounds exactly like a White Walker. As the stories go, when the two mated, she took his soul.
The Night's King then became the tyrannical ruler of the Night’s Watch, declaring himself king with his frosty queen by his side. It took a coalition between the Starks of Winterfell and Joramun, a King-Beyond-The-Wall, to end the Night’s King’s dictatorship at the Wall. He was finally taken out after a 13-year reign. Afterwards, it was discovered that the Night's King and his queen had been sacrificing... something to the Others. It is unclear what exactly it was, however, as all records of his reign were destroyed. His true name has been forever forbidden.
It seems unlikely that the Night King from Game of Thrones and the Night’s King from A Song of Ice and Fire are the exact same character. Although both come from the ancient past of Westeros, their origin stories are quite different.
George R. R. Martin himself made it fairly clear the two were not the same character, writing on his LiveJournal:
As for the Night's King (the form I prefer), in the books he is a legendary figure, akin to Lann the Clever and Brandon the Builder, and no more likely to have survived to the present day than they have.
Perhaps the most likely answer is that the Night King in the show is an amalgam of two or more book characters, which would not be unprecedented. The Night King may be a combination of book-version the Night’s King and the first White Walker.