Warning: This article contains SPOILERS for Game of Thrones' "The Long Night".
The biggest death in the latest episode of Game of Thrones has to be the Night King, stabbed by Arya, but did Bran know that this was coming all along? One detail points to the idea that Bran/Three-Eyed Raven saw this coming in Game of Thrones season 7 - which is why he chose to give Arya that particular Valyrian steel dagger (the same one that was used to try and kill him all the way back in Game of Thrones season 1).
Ever since Bran became the Three-Eyed Raven, he's been having visions - through the weirwood trees, he can see everything that has ever happened, and plenty of things that are currently happening. It's how he knew that Jon Snow is actually Aegon Targaryen, and how he could see some of what was coming for them at Winterfell. However, he's not always the most forthcoming about what he sees - he told no one that Jaime Lannister was on the way, instead waiting in person for Jaime to show up. And while he happily revealed that the Night King was going to come for him (and that no one had tried to kill him with dragonfire before), he hasn't always been known to shout his visions from the rooftops, instead keeping them a secret unless absolutely necessary. It definitely seems that Bran knew Arya would need Valyrian steel to kill the Night King, so he gave her a blade - long before anyone was planning to use Bran as bait in the first place.
Bran first gave her the dagger that she used against the Night King in season 7, episode 7, "The Spoils of War". It's here that Littlefinger brought the dagger back to Bran, telling him that it was the one used in the attempt to kill him. Later, in the godswood, Bran passes off that same dagger to Arya, in front of Sansa, telling her that it would be "wasted on a cripple." At the time, it seemed as though he was simply giving the only Stark fighter a useful weapon, but in retrospect, it seems like he may have known how she would be using it.
The dagger certainly comes with a history; it was the weapon used in an assassination attempt against Bran, but it was also part of the convoluted plot by Littlefinger to try and create animosity between the Starks and Lannisters. Since being gifted the dagger, Arya has been seen sparring with Brienne using it (and showing off the precise move that would be the end of the Night King), and she also used it to kill Littlefinger himself. It seems that this new blade could become her new favorite weapon (it's time she outgrew Needle, sadly), and that Bran knew from the start that she would need it - as Valyrian steel is one of the only things that will kill a White Walker, and that despite starting the night packing dragonglass, Arya ended up with just the dagger in the end.
If this is true, and Bran wasn't simply giving his assassin-sister a weapon as a way of telling her that he knew what she'd been up to with the Faceless men, it may have some big implications for the rest of the season. For one thing, it would mean that the Three-Eyed Raven can see a lot more than he is letting on. He claimed that he didn't know if dragonfire would kill the Night King, but if he saw Arya killing him with Valyrian steel, that may have been stretching the truth. However, it also means that, although Bran could be capable of seeing more than he lets on, he is also making a point of not interfering too heavily with the affairs of Westeros.
As Bran becomes the Three-Eyed Raven more and more, he is actively stepping into watching, rather than helping. This may be frustrating for Jon and Dany, but it actually helps with the show, preventing Bran from becoming an overpowered character, who would make it impossible for Jon to lose. Bran seeing that Arya would kill the Night King and choosing not to tell anyone (but to gift her the dagger anyway) suggests that he is going to become increasingly less helpful, as he truly embodies the Three-Eyed Raven, and even his family are left to fend without his powers as help. Thankfully, they've still got Arya.