The last dragon of House Targaryen was a green she-dragon hatched in the reign of Aegon III, small and stunted, with withered wings, one of the misshapen hatchlings that came from the eggs left after the Dance of the Dragons and never grew larger than a mastiff. Ser Arlan of Pennytree saw her as a boy and remembered her all his life. She died young in 153 AC, leaving a clutch of eggs that never hatched, and no dragon was born in Westeros again until Daenerys Targaryen walked into her husband's pyre a century and a half later.
The maesters and the wise disagree on what her death meant. Archmaester Marwyn holds that the Citadel itself conspired to see the dragons out of the world; the pyromancers of King's Landing say their spells began to fail the day she died; and Ser Arlan told the boy Dunk that the summers had grown shorter since, and the winters longer and crueler.