Blackfyre is a Valyrian steel sword, one of two in the possession of House Targaryen, the other being Dark Sister. The Targaryens brought the two swords with them to Dragonstone when they travelled there from Valyria a century before the Doom. During the War of Conquest, Aegon the Conqueror wielded the sword in battle. The sword was later wielded by many of the more martial Targaryen kings.
The sword was given by Aegon the Unworthy to his bastard son, Daemon, after he had proven his martial valour. The move was surprising, since it effectively removed the sword from House Targaryen's possession. Daemon founded his own lineage, House Blackfyre, taking the name of the sword. During the Blackfyre Rebellion, Daemon continued to wield the sword in battle, and fought an epic duel with Ser Gwayne Corbray of the Kingsguard at the Battle of the Redgrass Field. In that duel, Daemon wielded Blackfyre against Gwayne's own Valyrian blade, Lady Forlorn.
Ultimately, Daemon was killed in battle by his fellow Great Bastard, Brynden "Bloodraven" Rivers, who fought for House Targaryen during the Rebellion. After Daemon's death, the sword was rescued from the battlefield by another of the Great Bastards, Aegor "Bittersteel" Rivers, and taken into exile in the Free Cities. Once in the Free Cities, Bittersteel and Daemon Blackfyre's surviving sons founded the mercenary force known as the Golden Company. The sword Blackfyre remained in the hands of each successive generation of House Blackfyre until the line was wiped out with the death of Maelys the Monstrous during the War of the Ninepenny Kings. The sword's current whereabouts are unknown, with it believed to either have been lost or yet remain in the possession of the Golden Company.