Ariana Dumbledore was the sister of Albus and Aberforth Dumbledore. She died at approximately 14 years of age. According to Aberforth, when Ariana was attacked by three Muggle boys, it destroyed her emotionally and left her unable to control magic. She wouldnt use magic, but she couldnt get rid of it: it turned inwards and drove her mad, it exploded out of her when she couldnt control it. Ariana died when she was accidentally struck by a curse in a three-way duel between her older brothers and Gellert Grindelwald, the soon-to-be infamous Dark Wizard.