Arya Stark saved the entirety of Westeros when she plunged her Valyrian steel dagger into the torso of the Night King, shattering him and his White Walker posse to pieces. There's not much time to for the Starks to celebrate, however, as now the survivors of Winterfell will have to march south to fight Cersei. But are there any remaining supernatural threats from the North they should be worried about? Are the White Walkers really gone for good on Game of Thrones, or can consummate killer Arya Stark add an entire race to her kill count?
The nature of White Walker rules comes into play here, as killing a White Walker kills all of the wights and White Walkers that the initial White Walker has created. Since the Night King is the original White Walker, created by the Children of the Forest as a defense against the First Men, killing him means the rest of the White Walkers and every wight, be it north or south of the Wall, should die as well. However, this assumes that there is only one Night King, or the equivalent of what the Night King is. After all, it didn't seem to difficult for the Children of the Forest to make one Night King, so how can fans know that they didn't make another Night King that is just hanging out somewhere else?
If there isn't currently another Night King roaming around up north, it's entirely possible that someone who can make a new Night King still exists. While Bran saw some of the last remaining Children of the Forest die during his internship with the Three-Eyed Raven, there's no way of knowing if the entire race was truly wiped out, meaning that some CotF could still be kicking around and could make a new Night King. However, there is one other person who knows how to make a Night King, and who may have good reason to — the Three-Eyed Raven.
Reddit user aahdin suggests that the Night King's real beef with the Three-Eyed Raven is that he knows how to make another Night King. While the first Night King turned out to be incredibly evil, it's not clear if that was because he was intended to be or if perhaps the man that was sacrificed to become a Night King was evil himself and used his powers for the purposes of eliminating the human race. aahdin suggests that, in a last-ditch effort to defeat Cersei and her Golden Company, Bran may make a new Night King out of someone honorable, like Jon Snow, to defend Winterfell. Jon Snow would have to give up his humanity, but he's already living on borrowed time from the Lord of Light, so he's probably not long for this world anyway. Plus, it'd probably be pretty nice to have a Night King whose whole thing is never, ever, wanting to have any power over anyone. So, while the threat of the White Walker known to Westeros as the Night King has ended, the chances that White Walkers still exist or can be remade are higher than the Westerosi may expect.