The burning of Harrenhal occurred during Aegon's Conquest. Aegon the Conqueror used his dragon, Balerion the Black Dread, to melt the towers of Harrenhal, which killed King Harren the Black.
Harrenhal's walls were too high and too thick, and well-stocked enough to withstand a siege. During a parley outside the castle, Aegon offered to confirm Harren as Lord of the Iron Islands, provided he would yield. Harren refused, trusting in the strength of his castle, confident that the stone would not burn. Aegon warned him that the line of House Hoare would come to an end that night, at which Harren returned to his castle. To his men, he promised riches and the daughters of the riverlords to anyone who could slay Aegon's dragon, Balerion, in the upcoming battle.
When the sun had gone down, Aegon flew Balerion high above Harrenhal, before plunging down upon Harrenhal, burning the castle beneath him. All that was flammable, both suplies and ironmen, caught fire within the castle, while Harrenhal's stone towers cracked and melted. The rivermen outside observed that the towers glowed and melted like candles.
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