Dark Sister was the name of the Valyrian steel sword belonging to Visenya Targaryen, the sister-wife of Aegon the Conqueror. A great warrior, she used it in battle as well as riding her dragon, Vhagar, during Aegon's Conquest. Dark Sister is a comparatively thin and light blade, because it was the chosen weapon of a female warrior, though as it is made of Valyrian steel, it is still very strong and sharp.

By the reign of Viserys I, some sixty years after Visenya's death, Dark Sister passed to Daemon Targaryen, younger brother of King Viserys I Targaryen. It later passed to Aemon the Dragonknight and then to Brynden Rivers.

The sword was ultimately lost during the Blackfyre Rebellion.