House Blackfyre
The dragon reversed: the line of Daemon Blackfyre, King Aegon the Unworthy's legitimized bastard, who took the conqueror's sword for his name and half the realm to war for his claim. Daemon died at Redgrass Field twelve years past — but his sons live on in Tyrosh under Bittersteel's hard eye, and half the disappointed men of Westeros still drink to the black dragon.
The fuller tale
The wound the whole era nurses was self-inflicted by the crown itself: on his deathbed, Aegon IV legitimized every bastard he had ever fathered, and handed the ancestral sword Blackfyre — the blade every Targaryen king before him had carried — not to his trueborn heir but to his favorite natural son, Daemon. A sword is not a crown, but everyone in the hall understood exactly what the old king was implying about which of his sons looked more like a king to him, and the realm spent the next fifteen years finding out whether he had been right.
Daemon Blackfyre grew into precisely the man that gesture had promised — a peerless lance, married in time to a trueborn Targaryen princess, beloved by soldiers in a way his careful half-brother Daeron II never quite managed to be. The rumor that Daeron himself was not their father's true son gave Daemon's claim a second engine beyond simple popularity: if the whispers were true, a legitimized bastard with the old king's own blade in his hand had a better right to sit the Iron Throne than the crowned king did, and enough lords believed it, or wanted to, to make a rebellion out of it.
That rebellion broke on the Redgrass Field in 196 AC, where Daemon and two of his sons died fighting under the black dragon banner, brought down at the last by a bastard archer of the crown's own blood, Brynden Rivers, whose arrows found Daemon through the eye slits of his own borrowed helm. It is remembered as the realm's costliest single afternoon in a generation — and as the day the war supposedly ended, though the men who survived it never quite agreed that it had.
Daemon's half-brother Aegor Rivers, called Bittersteel for the color of his own blade, got what remained of the pretender's sons off the field and out of the realm entirely, settling them in Tyrosh under a watch as hard and unforgiving as his own reputation. There he built the Golden Company out of the rebellion's exiles and their sons — landless, unforgiven, and paid in the one currency Westeros never quite stopped owing them, which was grievance.
By the years Dunk and Egg wander the roads, the house exists mostly as a toast raised quietly in halls where the wrong lord thinks no one else is listening — 'the king across the water' — and as a name Lord Gormon Peake and men like him have never stopped doing arithmetic around. That the black dragon's heir can slip back into the Seven Kingdoms disguised as a hedge knight named John the Fiddler, gather half the realm's disappointed lords under one roof at Whitewalls, and very nearly try again, says everything about how unfinished 196 AC actually left things.
The page and the screen
None of House Blackfyre's story has reached a screen yet. The Hedge Knight season treats Redgrass Field strictly as backstory — a scar the characters mention rather than a battle the cameras show — and the Whitewalls conspiracy that gives the house its last real moment belongs to The Mystery Knight, a novella no announced season has been built from; readers currently know John the Fiddler's true face in a way viewers do not.