When Bran is discussing his history with the Night King on Game of Thrones Season 8 Episode 2, he mentions how the Night King marked him. In case you don’t remember what happened, we have a quick refresher.
Bran was marked by the Night King in Season 6 Episode 5, when Bran was using his visions to spy on the Night King and the White Walkers and see what they were doing. But the Night King apparently is a greenseer too and he saw Bran while Bran was watching him. The only other creature to be able to see Bran in a vision is the previous Three-Eyed Raven. So the Night King being able to see Bran was a big deal.
But it went even deeper than that. The Night King was able to touch and interact with Bran through Bran’s greensight. He touched Bran and marked him. Some theorized this would allow the Nightking to go through the wall, but this wasn’t the case. The Night King used an undead Viserion to go through the Wall and break it apart.
At the time that Bran was marked, the Three-Eyed Raven exclaimed, “He touched you!” Bran said, “I don’t know, maybe.” But Bran said that the Night King couldn’t get into the cave where they were hiding, and the Three-Eyed Raven said: “He can now.” Somehow the Night King marking Bran allowed him into the magical cave, eliminating the magic that protected it. It wasn’t needed for him to get through the wall, but he did need the mark to get into the cave.
Here’s that iconic scene:
Out of all the White Walkers and wights, only the Night King knew that Bran was spying on them. And his touch was incredibly painful when he marked Bran.
Now we know another reason why the Night King marked Bran. According to Bran, it was so he can track Bran and always know exactly where he is. So it looks like the Night’s Kings “greensight” might not be as advanced as the Three-Eyed Raven’s, who can essentially look almost anywhere and see what is happening. But it’s still advanced enough for the Night King to know when someone else is spying on him, like a greenseer.
When the Three-Eyed Raven (the one before Bran) was killed by the Night King, he had transferred all his knowledge and memories to Bran first. So now Bran is the Three-Eyed Raven, and the Night King is trying to kill him too.