Game of Thrones is full of mysterious characters, but one could make an argument for the Night King being the most inscrutable of them all. Since next to nothing has been revealed about him on the show—all we know is that he was a member of the First Men, transformed into a White Walker by the Children of the Forest after they stuck dragonglass in his heart—fans are left to theorize about who he was and, perhaps more importantly, what he wants. But one Redditor has concocted a pretty impressive theory about the Night King: that he's responsible for building the Wall.
User DanGliesac argues that the Night King built the Wall after the battle against him during the Long Night. This time period, which took place thousands of years before the War of Conquest (also known as the Targaryen Conquest), is when the White Walkers originally came to Westeros.
The idea is that since the Night King brings winter wherever he goes, and possesses great powers we don’t even know the extent of yet, he would’ve been the most likely creature to create the great ice wall. And his reasoning would’ve been simple: to keep out the Children of the Forest, the First Men, and anyone else who could be a potential threat to his people. While Westerosian history books claim that Bran the Builder was the one who built the Wall, it was Bran's people who wrote those stories. As such, the theorist concludes:
“After this happened, in an attempt to salvage the failure of not being able to kill the [Night King] I think the story was spread that the ‘good guys’ were the ones that put up the wall to stop the [Night King] from coming back when it was really the complete opposite.”
Fellow Redditors were quick to show their support for this theory, with one person adding that, “[The Night King] probably made a deal with Brandon the Breaker: he makes the wall, Brandon and the Children write the spells so he can’t get back.”
This theory would make sense with the recent reveal that the Night King is after just one person in the final season. In an interview with Entertainment Weekly, Vladimir Furdik—the actor behind the steely-eyed character—promised that, "People will see [the Night King] has a target he wants to kill, and you will find out who that is.”
If the Night King built the Wall for protection, it would make sense that he’s now only trying to break through it, thousands of years later, because of someone in particular. Who that person is, however, is still up for debate.