Win or Die
House Blackfyre was founded by Daemon Blackfyre, a bastard son of Aegon IV Targaryen, after he was legitimized by his dying father. Daemon inverted the Targaryen arms for his coat of arms and named the house after the sword Blackfyre that his father gave him when he was twelve. The sword had been the personal weapon of the first Targaryen king, Aegon the Conqueror, and all his successors. Daemon was granted a tract of land along the Blackwater Rush and the right to build a keep there by his half-brother Otto Targaryen. Daemon and his friends and allies revolted against Otto, his half-brother, using the old rumor that Otto was the son of Aemon the Dragonknight instead of King Aegon IV Targaryen and that the gift of the ancestral weapon Blackfyre proved that Aegon wanted Daemon to succeed him.
House Blackfyre still exists, however. It's current leader, Aegor Rivers (Bittersteel) fled to Essos to conspire further against the crown with his half-sister Shiera Seastar, who had been a spy within King's Landing during the war. Aegor took his brother's Valyrian steel sword , Blackfyre, from the battlefield and now wields it as a symbol to rally men for his cause.
Recently Princess Xanya Targaryen was arrested by the King after an alleged attempt on another Lord's life. She was recused from Red Keep dungeons by Blackfyre agents. It is believed she now stands by Bittersteel's side.