Warning: Spoilers follow for the Game of Thrones Season 7 finale as well as possibly for Season 8.
The penultimate episode of Game of Thrones Season 7 saw the Night King bolster his army with the addition of Daenerys' dragon who he converted over into one of the Army of the Dead at the end of the episode. As expected, the season finale saw the Night King riding the dragon, with the dragon breathing blue flames.
Now a new fan theory has hit the net which just might blow the doors off of Game of Thrones, as it theorizes who the actual identity of the Night King really is in the series. It's said the Night King might actually be none other than the Three-eyed Raven himself--Bran Stark!
The first potential evidence comes from a Game of Thrones Twitter account pointing out that Bran Stark and the Night King appear to be wearing the same clothes and resembling each other. Of course, that could simply be a nod to the "ying and yang' aspect of the Three-eyed Raven and the Night King being opposites (but not really at the same time) and at odds with one another, perhaps similar to Luke Skywalker wearing black in Return of the Jedi, which is a comparison to Darth Vader.
Bran and the Night King's outfits pic.twitter.com/Sikv9Jp4xe— Westeros Watch (@WesterosWatch) August 21, 2017
However, potentially the biggest evidence comes from the Game of Thrones Season 7 finale when the Night King attacks The Wall and opens a passage into the South. A Redditor put it forth that the Night King's Army of the Dead apparently forms the Stark Sigil! Woah!
Game of Thrones showed the Night King originally as man who was captured by the magical Children of the Forest, with one of the Children, Leaf, using Dragonglass to transform him into a White Walker, with the White Walkers used as a defense against man as they were at war.
Interestingly enough, in Game of Thrones, the Night King first appeared in one of Bran's vision as well! And we could also add that if Bran Stoke is the Night King, that would make him the true King of the North!
Regarding just how Bran Stark could be the Night King could obviously be explained by some sort of magic. Bran is able to visit the past, and as we know with Hodor, Hodor was directly affected by one of Bran's visions, which saw Hodor suffer a seizure where the young Hodor saw his future death, which became true with "Hold the door." Perhaps something similar happens with Bran Stark and the Night King at some point?