Valyrian steel is a type of metal that was formed back in the days of the Valyrian Freehold, a territory that encompassed most of Essos, as well as Dragonstone in Westeros, and was the main cultural and military power in the world before the Doom of Valyria, an unspecified cataclysmic event that brought about its downfall (roughly around 400 years before the events of the show). It is believed that dragonfire and magic was used in the forging of Valyrian steel, and blades made from it are lighter, stronger and sharper than those that can be made anywhere else, while theyll never lose their edge and require no maintenance, and have ripple effects that come from being folded over numerous times. The secrets of forging it were lost after the Doom, meaning that there is no one in the world now capable of creating a new Valyrian steel blade, although there are certain smiths capable of reworking it (as seen with Ned Starks Ice), but even this is an extremely difficult process, and a rare one given the relative scarcity of it