Warning: spoilers for season seven of "Game of Thrones," up to episode five, "Eastwatch." Read at your own risk.
The dead are coming for Westeros, and while there are many ways to kill living people, there's a limited number of ways to kill the dead.
Thankfully, Valyrian steel is one way to kill the White Walkers. Dragonglass is another.
This is proven when Jon Snow reduces a White Walker to bits in the battle at Hardhome in season five.
Valyrian steel is a metal that was forged in the days of the Valyrian Freehold. Valyrian steel remains sharp forever. It's also recognizable by its intense sharpness and light weight.
Since the Doom of Valyria, most Valyrian steel weapons are heirlooms in the great Houses of Westeros. Over the past seven seasons, several characters have stolen, stumbled upon, or been gifted Valyrian steel blades. Valyrian steel is also hard to forge since it's not like other metals, hence the rarity of the weapons made out of it. Only a few smiths in Westeros can do it out of existing weapons. But the method of forging completely new blades was lost after the Doom of Valyria, and allegedly involved dragons and magic.
Here is everyone on "Game of Thrones" who has a Valyrian steel weapon and can kill the White Walkers:
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Jon Snow: Longclaw
Longclaw is the ancestral sword of House Mormont. Lord Commander Jeor Mormont originally leaves it to his son, Jorah, before he leaves for the Night's Watch. But Jorah leaves it behind in shame when he leaves Westeros. Then Jeor gives it to Jon Snow, basically as a thank you present for saving him from a wight.
Brienne of Tarth: Oathkeeper
Jaime Lannister gives Brienne Oathkeeper when he sends her out to complete her mission of bringing the Stark girls home to Winterfell. Oathkeeper is made out of Ned Stark's sword, Ice. At the beginning of season four, Tywin has it melted down into two blades.
Jaime Lannister: Widow's Wail
After Joffrey's death, Jaime takes Widow's Wail. This sword is also made out of Ned Stark's sword, Ice, which was very big.
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