In Season 6 Episode 5 titled "The Door" of Game of Thrones, the Night King marks and then pursues Bran.
Why? Is it because He sensed that, like the Three-Eyed Raven, Bran could be a powerful agent in the coming war against the White Walkers?
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Sign up to join this communityIn Season 6 Episode 5 titled "The Door" of Game of Thrones, the Night King marks and then pursues Bran.
Why? Is it because He sensed that, like the Three-Eyed Raven, Bran could be a powerful agent in the coming war against the White Walkers?
We do not know if Night's King felt any special power in Bran. However we know why he touched Bran as explained by the three-eyed Raven to Bran in the same episode:
(gasps)
(gasps)
(screams)
Bran: He saw me, the Night King! He saw me!Three Eyed Raven: He touched you.
Bran: I don't know. He was close, but--
Three Eyed Raven: He touched you. He knows you are here. He'll come for you.
Bran: But he can't get in.
Three Eyed Raven: He can now. His mark is on you.
So as explained by TER, After touching Bran, Night's King knew where Bran physically was and whatever protection their abode had against the Walkers was now gone because Bran had Night King's mark on him and therefore made him aware of his exact location.
Since TER also says that The Night's King was now gonna come for Bran, we can presume that Three Eyed Raven assumed that Night's King had discovered Bran's power (Because who wouldn't, after all Bran was there in some weird non-Physical way where only NK could see him and his Army couldn't) so maybe he came to the same conclusion that this must be the new champion of Life. Since NK explicitly marked Bran, we can also assume that he somehow knew the exact nature of Bran's presence there and knew that he had to mark him to find out where he actually was.
So in conclusion:
Because touching Bran meant the Night King could assault the Weir Tree where the Children of the Forest lived with the Three Eyed Raven.
After the vision, Bran woke up to find a mark where he was touched. The Three-Eyed Raven warned him that he must leave, as the Night King can now not just find them but also enter the cave to destroy them.
The Night King's mark upon Bran negates some of the magic protecting the cave.
We don't know if the Night King fully understands Bran's powers or what Bran might be able to do in the war against the White Walkers.
What he does seem to know is that touching Bran gives him an opportunity to destroy the Raven.
As we learn in Season 8, Episode 2, the Night King is after Bran because he is always after the Three-Eyed Raven, whomever that may be...
Bran: He'll come for me. He's tried before, many times, with many Three-Eyed Ravens.
Sam: Why? What does he want?
Bran: An endless night. He wants to erase this world and I am its memory.