SPOILER ALERT: This article contains major spoilers from Fantastic Beasts and Where To Find Them. It's meant to be read after you've seen the movie (or at the very least, read the screenplay book.)
With Fantastic Beasts and Where To Find Them in theaters, Harry Potter fans are once again reconnected to J.K. Rowling's incredibly rich and magical world. Those hardcore fans who were around when the books were releasing likely remember all of the theorizing and obsessing over plot points and character arcs between books. Merlin knows, we had plenty of time between books to pick through the canon in search for clues, and J.K. Rowling often left breadcrumbs. So it's no surprise that fans are already using the new information provided by Fantastic Beasts, and finding ways to connect it to the information we learned in the Harry Potter books.
Fantastic Beasts introduced a new magical being to Harry Potter canon in the form of the Obscurus, and Harry Potter Alliance's Jackson Bird (via ScreenCrush) hypothesizes that this specific condition, which befell Ezra Miller's character Credence, may have also applied to Ariana Dumbledore.
Was Ariana Dumbledore an Obscurus?
It makes a lot of sense, when we take into account what we know about Ariana, and Grindelwald, for that matter.
What happened to Ariana Dumbledore?
Dumbledore's younger sister was abused by a bunch of muggle kids when she was just six years old. The ordeal caused her to become emotionally and magically unstable. Albus' father took vengeance on the kids who hurt Ariana, landing him in Azkaban. Albus' mother Kendra kept Ariana at home, out of view from the magical community, sparking rumors that she was a squib, and that Kendra was ashamed of her. In Deathly Hallows, Dumbledore explains to Harry that the real reason Ariana was kept hidden was because they feared she'd be seen as a threat to the International Statute of Secrecy and shipped off to St. Mungo's.
So, Ariana was kept hidden away, and at fourteen, one of her outbursts resulted in her mother's death. This, according to both Albus and Aberforth Dumbledore, though neither of them were there at the time. Dumbledore's issues with the International Statute of Secrecy stemmed from the tragedy surrounding his sister. And that proved to be a binding ingredient in the friendship he quickly formed with Grindelwald not long after his mother died. Grindelwald wanted to live in a world where wizards were dominant over muggles. Dumbledore wanted to live in a world where his sister didn't have to live in hiding. And probably a world where she wouldn't have been abused by those muggles in the first place.
A few months after Kendra died, Ariana was killed in the crossfire of a three-way fight between Albus Dumbledore, his brother Aberforth and Gellert Grindelwald. So it seems very possible that what Aberforth describes (in Harry Potter and the Deathly Hallows) as Ariana's "rages" were the same kind of outbursts we saw from Credence. But let's poke at this theory a bit and see how well it holds up.
Ariana and Credence don't have the same background, but there are some significant common threads. Ariana was raised in a loving family environment, unlike Credence, who was abused by his adoptive mother, and raised to believe that magic is evil. The implication in Fantastic Beasts is that this mistreatment led him to stifle his magical abilities, and between that and not being trained at all, Credence became (or produced?) an Obscurus.
In Ariana's case, the abuse she suffered at the hands of the muggle kids left her emotionally scarred, magically unstable and prone to outbursts or "rages." Did these outbursts manifest in the form of an Obscurus? What little we know about her outbursts wouldn't rule that out, especially when we consider that one of them resulted in her mother's death, when Ariana was just fourteen.
And that age situation is the only real hole in this theory...
It seems clear that Gellert Grindelwald is in New York looking for an Obscurus. That situation is more and more apparent when you take into account Graves' behavior and appearances throughout the film. We first see GrindelGraves investigating a scene where something described as a dark wind recently tore through a building. Graves is clearly looking into the situation. We're meant to believe it's because he's there on behalf of MACUSA, but in retrospect, it seems just as likely that his investigation of that scene ties in with his relationship with Credence. He's looking for the troubled kid. And, of course, he's barely able to contain his interest in Obscurus when he discovers one contained in Newt's case, which is another big giveaway.
So, it's evident Grindelwald has an interest in Obscurus, which may or may not stem from his connection with Ariana Grindelwald years ago. But Ariana Dumbledore was fourteen when she was killed. Assuming Grindelwald knew (or learned) what she was, wouldn't he have known that a child could be in their teens and still be an Obscurus? And if that's the case, why was he so quick to dismiss the idea that Credence is an Obscurus?
It's a technicality that could be overlooked, I suppose. Credence is clearly older than fourteen, though I'm not sure exactly how old he's supposed to be (Ezra Miller is in his early twenties). And I'm kind of inclined to tentatively give Grindelwald a pass for overlooking Credence, even if he might have known better... unless/until J.K. Rowling rules this Ariana Obscurus theory out. In the meantime, the connection between Grindelwald and Dumbledore is too relevant not to consider Ariana and the circumstances surrounding her life and death as potentially a major factor in this series.
That brings us to the last point that should be brought up concerning Ariana Dumbledore. Who killed Ariana? There were three wizards in the room when she died, and one of them most likely knew what happened to her...