Balerion

Of all the dragons of House Targaryen, none is greater than Balerion, called the Black Dread, whose name echoes through A Song of Ice and Fire from A Game of Thrones onward and whose full history is told in Fire and Blood. Hatched in Valyria before the Doom and carried to Dragonstone by the fleeing Targaryens, Balerion grows so vast that the shadow of his wings can cover a town, and his fire burns as black as his scales. Aegon the Conqueror rides him in the War of Conquest, where the Black Dread melts the towers of Harrenhal, the greatest castle ever raised, and it is Balerion's flame that forges the Iron Throne from the swords of the Conqueror's enemies. In the present day, dragons are gone from the world, and Balerion's immense skull, largest of them all, rests in the cellars beneath the Red Keep.