Summerhall

Aegon V ÔÇö the boy who held Dunk's horse, now an old king who never stopped trying to help the smallfolk ÔÇö attempted something at Summerhall involving wildfire, seven eggs, and a desperate hope of waking dragons again. The pleasure palace burned with the king, his heir, and Ser Duncan the Tall inside it. Rhaegar Targaryen was born that same night, and was said to have been sad ever after.

The fuller tale

Aegon V was the fourth son of a fourth son and should never have come near the throne. He got there through a plague, a Great Council, and a brother who chose a maester's chain over a crown, and he came to it having spent his youth on the roads of Westeros holding a hedge knight's horse. He knew, in a way no other Targaryen did, what the realm looked like from underneath.

He spent his reign trying to do something about it ÔÇö laws for the smallfolk, rights the lords had never granted, courts they could reach ÔÇö and the lords fought him at every step. He is called Egg by the histories that like him and the Unlikely by the ones that are being careful. What he learned is that a king without dragons could be refused, repeatedly, by his own bannermen.

Which is, most likely, why an old man came to his summer palace at Summerhall with seven eggs, a company of pyromancers, wildfire, and a woods witch's prophecy, and tried to wake dragons out of stone. What exactly was attempted is not recorded because everyone who planned it died in the doing. The palace burned. The king burned, and his eldest son, and Ser Duncan the Tall of the Kingsguard, who by every account went back into the fire for him.

Born in salt and smoke

Rhaegar Targaryen was born at Summerhall that same night, while it burned. He was said to be a sad man ever afterward, and he went back to the ruin in later years to sit among the stones with a harp. Whatever he read about himself in the Citadel's books, he read it in the light of having been born in that fire.

The Targaryens tried once more to be what they had been, and it killed a king, a prince, a Lord Commander and a palace, and produced one child who spent the rest of his short life trying to work out what it had been for.

Elsewhere in the wiki

 See the world as it stood in 259 AC 

What became of it ÔÇö if an ending is written, it is here. Spoilers.

The chronicle records no ending here ÔÇö or none the maesters have set down.