In the books, the power of Valyrian steel is elaborated on a bit more. It's a rare metal that was forged by old Valyrian empire (the Game of Thrones equivalent of the Roman empire). The process to create Valyrian steel involves blood magic and dragon fire, which imbues it with special properties: notably, it is much lighter than regular steel, and it has a slightly rippled texture that prisms in the light. Since Valyria fell, the knowledge of creating Valyrian steel was lost, so no new weapons have been made from the material. Some Valryian steel weapons still exist in Westeros and are considered family heirlooms in addition to serving as battle weapons. Ned Stark's sword Ice is one such weapon, and it was melted down and reforged into Widow's Wail and Oathkeeper by the Lannisters. Valyrian steel is especially important because it can kill the White Walkers , as we saw when Jon Snow slayed one of them with his sword Longclaw.