The Sword of the Pendragon was one of the Things of Power, and known as Eirias, which means "blazing". The crystal sword was ancient, used by the heir of King Arthur to cut the ripe branch of the Midsummer's tree. In the presence of the Dark, it burns with blue fire. The sword was crafted by Gwyddno Garanhir for the Light, but because it is a double-edged sword, it could also be used by the Dark.
The Sword of Pendragon is the fourth of the Things of the Power, and the most powerful. It always point to the east, because it represents the sunshine.
Fate of the Sword
King Arthur Pendragon takes Eirias away with him to whatever final destination all the Old Ones are headed to.