Bran messed up. There's no getting around the fact that Bran's decision to go greenseeing unsupervised was the worst decision made in Westeros since Robb Stark thought that going against his word and marrying Talisa Maegyr would end well. Robb Stark's decision didn't go well, and the consequences of it have rippled throughout Westeros since, but that may be nothing compared to the lasting implications of Bran going into the past on Game of Thrones . While the Starks may be good and honorable people, they really can mess things up for everyone awful easily. And, in the wake of Bran's decisions in "The Door," things are going to be very different.
We now know that Bran is, somehow, able to affect the past — but that doesn't necessarily mean he can change it. Hodor (it's okay if you need a moment here, the pain is still fresh) was always Hodor for as long as Bran knew him. Bran may be the reason that Hodor is Hodor, but he didn't necessarily change the past — he merely had a role in causing it. While the revelation of Bran's ability to participate in the past is pretty massive, it doesn't really change much. What does change things, however, is the mark that Bran carries on his arm. The mark of the Night's King allowed the White Walkers and the wights to pass through the magic barrier of the cave that held the Three-Eyed Raven. What does this have to do with Westeros? It could mean that, after thousands of years, The Wall could finally fall.
Bran Can Lead The White Walkers Anywhere
The White Walkers were able to infiltrate the magical barrier protecting the Three-Eyed Raven and those inside the cave. Not only did Bran's mark lead the White Walkers to the cave, but allowed them to break through the magical protection surrounding it. It's hinted in A Storm of Swords that The Wall itself is imbued with magic that prevents the wall from being overrun by beings of magical nature like the White Walkers. Bran passing south of the wall could allow The White Walkers to pass through the wall and wreck their havoc on all of Westeros. This is very, very bad.
Bran Could Discover The Truth About Anything And Everything.
There are a lot of lies in Westeros... And Essos... and just about everywhere. As proven in the play that is performed in "The Door," popular opinion often trumps truth and people live lies their whole life. However, now that Bran is free to greensee anywhere his heart desires, he could learn the truth about many of the secrets embedded in Westerosi History. Bran was able to witness the truth behind the fight at the Tower of Joy — that Ned Stark was not as honorable as he said he was — and Bran could soon witness any number of earth-shattering revelations. Bran may be able to affect the past, but his ability to observe it is arguably more important. Get ready for that R + L = J reveal, everybody. It could be coming any day now.
Bran Has Placed A Huge Target On Himself
The White Walkers did not like The Three-Eyed Raven. It's unclear why, but there was some pretty severe beef between the two. Now that The Three-Eyed Raven has bestowed his power unto Bran, the young Stark is effectively Public Enemy No.1 for The White Walkers. This puts him, Meera, and anyone that they might encounter (perhaps a long-forgotten face like Benjen Stark or someone with some especially cold hands) in severe danger. While it's clear that being the Three-Eyed Raven came with some massive power, the extent of that power isn't entirely clear. Bran could warg into a dragon, he could communicate with those in King's Landing through mystical means, he could even set forth motions in the past that affect the present — while the scope of Bran's new power isn't clear, what is clear is that the White Walkers are scared of his power.
Bran's trip to the Three-Eyed Raven may not have seemed like the most important plot in the series, especially since the storyline effectively took Season 5 off for vacation, but now it feels as though Bran has ascended up the rankings to become the most important character in all of Game of Thrones. Bran's power is immense, and the implications of that power are even larger. Bran has hopefully learned from his biggest mistake, because if not he could end up handing Westeros over to the White Walkers and cause the extinction of mankind. Now is probably a really good time for anyone in King's Landing to start heading to Essos.
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