When Ariana was six years old, three Muggle boys from Mould-on-the-Wold saw her doing magic, and attacked her. She was never the same again; her powers were somehow trapped inside her, highly unstable and out of control, and it drove her insane (DH28). After the incident she refused to use her magic but she couldn’t remove her magic either. At times her magic would explode out of her in uncontrolled and unpredictable ways.
After the attack on Ariana Percival Dumbledore, Ariana’s father, attacked the boys that injured his daughter and was sent to Azkaban for his actions. It was then, with her husband in prison and her family fractured, that Kendra Dumbledore moved her family to Godric’s Hollow. Kendra decided to keep her daughter’s condition a secret and once they had moved the family kept mostly to themselves. While Albus, and later Aberforth, would attend Hogwarts, Ariana remained hidden, untrained, and her magic uncontrolled. The Dumbledores claimed that Ariana had a sickly, frail disposition; many people suspected that she was in fact a Squib (DH8).
Her brother Albus, who graduated in 1899, intended to set out a tour of the wizarding world with his friend, Elphias Doge. Tragically, it was then that his sister Ariana, 14 years old at the time, killed their mother Kendra in an accidental burst of uncontrolled magic. The family told everyone that Kendra was killed by a backfiring charm. And instead of traveling the world and launching the beginning of what was surely to be a great career, Albus reluctantly stayed in Godric’s hollow to care for his unstable sister.