Aegon the Unworthy legitimizes his bastards
Dying, bloated and spiteful, Aegon IV signed a decree legitimizing every natural child he had ever fathered ÔÇö an act one maester called the single most destructive thing any Targaryen ever did with a pen. Chief among them was Daemon Blackfyre, given the ancestral Valyrian sword of the kings. Five rebellions grew out of that afternoon.
The fuller tale
Aegon IV is called the Unworthy by common consent of everyone who has ever studied him, which for the Citadel is close to a formal verdict. He ate himself into a wreck, quarrelled with his own son, kept mistresses in numbers his court kept a list of, and produced children by most of them.
On his deathbed, bloated past moving, he signed a decree legitimising every natural child he had ever fathered. All of them. At once. One maester wrote that it was the most destructive thing any Targaryen ever did with a pen, and he was not exaggerating for effect.
The eldest and most dangerous was Daemon Waters, a brilliant boy who had been given Blackfyre ÔÇö the ancestral Valyrian greatsword of the kings, carried by Aegon the Conqueror ÔÇö while the trueborn heir got nothing. He took the name Blackfyre from the sword and the black dragon on red for his arms, and a great many people who disliked the trueborn heir noticed that the sword had gone to the other one.
Why the sword mattered
Legitimising bastards created claimants. Giving one of them the sword created a claim. Aegon the Conqueror's own blade in the hand of a legitimised bastard is a piece of political theatre that no amount of law could argue with, and the Blackfyres used it for five rebellions across seventy years.
The kings who followed spent their reigns fighting a war their father had started as a spiteful joke, and the last of it does not finish until an exile dies on a bridge in the Disputed Lands a lifetime later.
Elsewhere in the wiki
 See the world as it stood in 184 AC What became of it  if an ending is written, it is here. Spoilers.
The chronicle records no ending here ÔÇö or none the maesters have set down.