Dark Sister was one of two Valyrian steel swords in the possession of House Targaryen, along with Blackfyre. Whilst Aegon the Conqueror wielded Blackfyre during the War of Conquest and afterwards, Dark Sister was bequeathed to his warrior-sister Visenya.
Dark Sister was wielded by several famed warriors of House Targaryen, including Prince Aemon the Dragonknight, before it was bequeathed to Brynden Rivers, known as 'Bloodraven', for his loyal service to King Daeron II Targaryen, despite his known preference for the bow. Bloodraven wielded the sword at the Battle of the Redgrass Field during the Blackfyre Rebellion: after slaying Daemon Blackfyre with arrows, he crossed swords with his hated half-brother 'Bittersteel' and drove him from the field despite the loss of his eye. Bloodraven continued to wield the sword after the war and whilst executing his office as Hand of the King to King Aerys I.
The eventual fate of the sword is unknown. Aegon V sent Bloodraven to the Wall to serve in the Night's Watch, eventually becoming Lord Commander, but it is unclear if he took the sword with him or not.