Tonight on Game of Thrones, Daenerys and Jon came face to face with the hordes of the icy undead. Countless soldiers perished trying to take on the army of wights, but it was all to no avail. At one point it seemed like Dany might have been able to end the whole thing with a single "dracarys," but the Night King remained stubbornly unmelted by Drogon's fiery breath. So how do you kill the Night King on Game of Thrones?
The Night King didn't get involved in the fighting until the battle was already underway. He waited for his soldiers to demolish the opposing forces before he swerved in on his shiny new ice dragon Viserion to do some real damage. Jon was able to knock him to the ground eventually (and was unseated in the process himself), which was when Daenerys attempted to dispatch him with Drogon's flames. Fire has been one of the things proven to kill wights, but it seems to have little to no effect on White Walkers. When the flames dissipated, the Night King was practically smirking.
Luckily, before Episode 3 was over, one character would figure out exactly how to kill the Night King. It just wouldn't be Jon Snow, (obviously, he knows nothing).
Jon was willing to give it his all, but the Night King was two steps ahead of him as always. Jon was racing towards him with his Valyrian steel blade in hand when the King used his powers to raise the dead — and I'm talking, all of the dead. He revived every fallen soldier, every deceased defender, every Stark in the Winterfell crypts. At that point, Jon was a little too busy to find out how to take him out. He had to figure out how to stay alive instead.
The Night King has been after Bran for a while now, so while everyone else was busy fighting, he made a beeline for everyone's favorite creepy tree boy. Theon had spent the entire skirmish acting as Bran's protector, but when the Night King arrived, he didn't even get a slice in. He was impaled before anyone could blink, leaving a wide open path for the King to kill Bran. For one long moment, it didn't seem like viewers would ever be free of the villainous dead.
The Night King was reaching for his weapon when Arya Stark flung herself out of the darkness with her knife in hand. The King caught her by the throat, but Arya used quick thinking and even quicker fighting skills to get her blade from one hand to the other. Then she plunged it into his stomach. In a flash, the Night King was destroyed and the entire army of the dead perished with him. But how?
Arya's blade was actually a very significant one: it looked like the same Valyrian steel knife that was used in the assassination attempt on Bran all the way back in Season 1. Since then, it had passed into Littlefinger's hands before he gave it back to Bran. Then Bran handed it over to Arya in Season 7, likely knowing (because he is an all-seeing tree boy) that she would one day use it to save the world.
Valyrian steel was all it took to take out the Night King — with a little help from a mysterious Stark who doesn't believe in spoilers.