Arthur Eld was the ancient King of All-world. He is the only person before or since to have united the entire world. He was a warrior of the White. The line of Deschain (the line of such gunslingers as Steven and Roland) is descended from one of his forty gillies. The Crimson King is also descended from Arthur Eld although it is unclear how this came to be.
He is the King Arthur of legend. The sword Excalibur was melted down to make the sandalwood guns that Roland inherited. It is rumored that only someone who carries Excalibur or another sign from Arthur Eld may enter The Dark Tower.