This post contains spoilers from Game Of Thrones Season 7, Episode 5. With Bran's return to Winterfell, Sansa and Arya now have a key ally in Jon's War Against The Dead, even though no one actually realizes it yet. But it wasn't going to be long before Bran's continuing meddling in the affairs of the Dead caught the attention of the Night King, especially with his only half finished training. And that's exactly what happened when Bran sees the Night King on Game Of Thrones.
Unlike last time, Bran isn't above the Wall in a tree coma with the Three Eyed Raven. For that we will all count our blessings, because no one at Winterfell is ready for the Night King to arrive uninvited at this stage of the game. Also, despite the Night King looking right at Bran with a gaze that says "screw off kid," he's also on his own mission right now. He doesn't have time for some Baby Stark playing at Raven-hood.
He's heading somewhere very important. A place where he can cross the Wall.
The good news for the Starks: Bran now knows this and Sansa believes him enough at this juncture that his message will get sent.
The bad news for Jon Snow: The Night King knows Bran saw him, and can probably guess there will be trouble along the way. No, he doesn't know things, like "Jon is at Dragonstone" so it will take a while for Sansa to get word to him -- but she will, since she's heard Jon go on and on about the Night King enough.
Nor does he know that Jon could come armed with a few dragons if Dany agrees to it.
But an army that knows the enemy is aware of them is an army at the ready And considering how badly Hardhome went, losing that element of eventual surprise is a bummer.