HBO
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.
Here is everyone on "Game of Thrones" who has a Valyrian steel weapon and can kill the White Walkers: