When arianna was 6 years old, she was attacked by three muggle boys who saw her magic. Since then, her spirit has become extremely unstable and unable to control her magic. Father was arrested for putting a spell on those muggle boys in revenge. He didn't tell the Ministry of Magic why he attacked Muggles, because it would lead to arianna being locked up in the hospital for life. 1August, 899, my brother Albus met his new friend Grindelwald and neglected to take care of his sister, which made him very angry. The three men had a duel. Arianna tried to stop it, but was hit by a spell shot by someone and died unexpectedly. In Deathly Hallows, Aberforth tells us the whole process of arianna's illness: Deathly Hallows Original: Chapter 28 The Lost Mirror My sister was attacked by three Muggle boys when she was six years old. I saw her magic through the hedge in the back garden. She is still a child and can't let go, even a wizard of that age. The boys were frightened by what they saw. They squeezed through the hedge, and my sister couldn't tell how magic changed, so she lost control and tried to stop the little monster from changing magic again. ""what I did ruined her and never returned to normal. Unwilling to use magic, but unable to get rid of it. Magic is deeply rooted in people's hearts and drives you crazy. When you can't control it, magic will attack you Sometimes it's weird and dangerous, but most of the time it's cute, timid and harmless. ""Father has gone to settle accounts with those bastards, "Aberforth said." He taught me a lesson and finally got into Azkaban. Never said why he did it. If the Ministry of Magic knows about arianna's illness, he will be imprisoned for life in San Mungo Hospital. She will be regarded as a serious threat to the international secrets law, because she is insane and her inner magic will erupt if she can't control it. ""You must be safe and hide. I moved home and lied that I was sick. My mother is responsible for taking care of her and trying to make her calm and happy. " .