In the massive battle at Hardhome, Jon Snow goes head to head with one of the White Walkers. The creature shatters when Jon strikes it with his sword, Longclaw, the ancestral weapon of House Mormont that is forged from Valyrian steel.
In the seventh season, Bran gives Arya the dagger under the weirwood tree — the same Valyrian steel dagger that is used in Bran's assassination attempt, slices her mother's hands, and is seen on her father's desk in the tower of the Hand . . . the same dagger that she uses to execute Littlefinger. This dagger, like its new owner, has a long and complicated storyline. It is only fitting that it should be used to take down the most formidable enemy in the Seven Kingdoms.