The Night King theory is based on the idea that Bran can time-travel and warg . The theory suggests that Bran swoops back in time to the day that the Children of the Forest created the White Walkers with a shard of dragonglass. However, through a happy accident, the Children instead stab Bran, who then becomes the Night King. There are a few details that support this theory — one gif shows Bran and The Night King in the same scene north of the wall in strikingly similar clothing. Another detail you might have missed is that the White Walker army formed the Stark sigil over a sheet of ice in the season seven finale. If the theory is true, it means that the Night King is the Darth Vader of Westeros: He's evil, but he might just have some good left in him.