From A Wiki of Ice and Fire
Dawn is the ancestral greatsword of House Dayne, who bestow the title Sword of the Morning on the sword's bearer. It is said to be made from metal forged from the heart of a fallen star. Its blade is as pale as milkglass. The name of the Daynes' castle, Starfall, and their arms, featuring a white sword and falling star, both reference Dawn. The blade is said to be just as sharp as Valyrian steel.