One of the more mysterious yet significant characters in the Harry Potter world is Ariana Dumbledore, the younger sister of Albus and Aberforth. Although tragically killed during a duel years earlier, her death would have far-reaching, intimate repercussions on her brothers, their relationship, and maybe the wizarding world at large.
The Harry Potter series has no shortage of evil antagonists. Voldemort, the Death Eaters, their multitudes of minions. And yet, ironically, one of the most egregious and prominent acts of evil perpetrated in the wizarding world was committed by three Muggle boys against Ariana when she was just six years old.
‘When my sister was six years old, she was attacked, set upon, by three Muggle boys. They’d seen her doing magic, spying through the back garden hedge: she was a kid, she couldn’t control it, no witch or wizard can at that age. What they saw scared them, I expect. They forced their way through the hedge, and when she couldn’t show them the trick, they got a bit carried away trying to stop the little freak doing it.’
This attack caused Ariana’s magical powers to “turn inwards,” meaning the emotional and mental scarring she experienced rendered her unable to harness her power in a safe and productive way. In Harry Potter and the Deathy Hallows, Aberforth describes his sister: “She wouldn’t use magic, but she couldn’t get rid of it; it turned inward and drove her mad, it exploded out of her when she couldn’t control it, and at times she was strange and dangerous. But mostly she was sweet and scared and harmless.” Ariana ended up killing her mother in a fury,
This story led Potterheads to deduce that Ariana is in fact an Obscurial, a dark force that is created when a child is made to “repress their talent through physical or psychological abuse.” This powerful parasitic energy would find some manner of release, often manifesting as “a separate entity that can erupt in violent, destructive fury.” It is widely believed that this is exactly what happened when Albus and Aberforths mother Kendra is killed just after Albus graduates school.
“‘Then, when she was fourteen … see, I wasn’t there,’ said Aberforth. ‘If I’d been there, I could have calmed her down. She had one of her rages, and my mother wasn’t as young as she was, and … it was an accident. Ariana couldn’t control it. But my mother was killed.'”
Albus returned home instead of traveling the world as planned, where he came to seriously resent his sister for their mothers death and what he saw as his imprisonment.
‘I resented it, Harry… I was gifted, I was brilliant. I wanted to escape. I wanted to shine. I wanted glory. Do not misunderstand me,’ he said, and pain crossed the face so that he looked ancient again. ‘I loved them. I loved my parents, I loved my brother and my sister, but I was selfish, Harry, more selfish than you, who are a remarkably selfless person, could possibly imagine.
‘So that, when my mother died, and I was left the responsibility of a damaged sister and a wayward brother, I returned to my village in anger and bitterness. Trapped and wasted, I thought! And then, of course, he came …’
…Of course came the brilliant Gellert Grindelwald, who was on track to become the most evil wizard ever until Voldemort filled that post. Gellert and Albus became inseparable, sharing their ideas for a new wizarding world order. They began to make plans which Aberforth quickly tried to stop, arguing this would ultimately leave Ariana abandoned. This is how the fateful duel between Albus, Aberforth and Grindelwald erupts. Ariana intervenes, and is killed by what may or may not have been a stray spell.
SO. Back to the Reddit theory at hand (posited by Redditor shazzymcgee) which suggests Grindelwald was obsessed with the obscurus because Ariana was more powerful than him, and also that Albus was indeed responsible for his sister’s death: