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Ariana Dumbledore was the youngest of three children of Percival and Kendra Dumbledore, born in 1885 in Mould-on-the-Wold, a wizarding village in England. She was the younger sister of Headmaster Albus Dumbledore. When she was six years old, she was spotted practicing magic by a group of Muggle boys who assaulted her when she couldn’t show them how to do the things she was doing. As a result of this attack, she was left emotionally and mentally scarred, and thus unable to control her magic. Her father was sentenced to life in Azkaban for the crime of seeking revenge on the Muggle boys, as he refused to give a defense for his actions. He didn’t want to reveal what happened to Ariana because he feared that she would be sent to St. Mungo’s if her condition was discovered.
Kendra Dumbledore, her mother, moved the family to Godric’s Hollow and kept Ariana secluded from the outside world in order to protect her. Many people weren’t aware that the Dumbledores even had a daughter. Those who did know about her thought she might be a squib, as no one saw her and she never went to Hogwarts. Some believed that she was actually an Obscurus, whose magic was stifled due to the attack on her as a child.

Ariana was a sweet and kind child. However, when she couldn’t control her magic, she became a danger to those around her. When she was just fourteen years old, she had an episode where her magic spun out of control. Her brother Aberforth, who was the only one who could calm her down in these instances, was not at home at the time and, unfortunately, she caused an explosion that resulted in the death of her mother.

After the death of their mother, Albus Dumbledore was granted guardianship over Ariana. At the time, Albus Dumbledore was friends with Gellert Grindelwald and the two planned to take over the wizarding world together. When Aberforth confronted them about their plans and what would happen to Ariana while they were traveling, a three-way duel broke out. During this duel Ariana tried to stop it, however, she had no control over her magic and was inadvertently killed by a stray curse. There is no consensus as to whose spell hit her. After this event, Grindelwald fled, and his friendship with Albus Dumbledore eventually ended.

A portrait of Ariana (painted in 1899, two months prior to her death) hangs in Aberforth’s room in the Hog’s Head Inn in Hogsmeade that concealed a secret passage to the Room of Requirement in Hogwarts. This passage appeared during the Second Wizarding War, when students began to use the Room of Requirement as a central location to fuel their fight against the new Hogwarts administration after the Death Eaters took over under Headmaster Severus Snape. It was used by Harry Potter, Hermione Granger, and Ronald Weasley (as well as other members of Dumbledore’s Army and the Order of the Phoenix) to enter Hogwarts immediately preceding the final battle in 1998.