I’ve written before about Ariana as an obscurial, and I still think it’s a strong possibility – but as much as she has in common with Credence, there are also several major differences between them. If an obscurus forms over a long period of time, as a result of repeatedly trying to suppress one’s magic, maybe they’re not as clear-cut as they seem. It seems like there would be some gray area between “well-adjusted magical child” and “angry dark cloud of destruction”, and that someone who becomes an obscurial would first go through that gray area rather than suddenly becoming one overnight. Here are my thoughts on what that might look like:
Normal Childhood Magic
It’s always portrayed as normal that magical children will not have control of their magic until around the age of eleven. Only in rare cases are they able to use it intentionally, and they may or may not even suspect that what they’re doing is magic. However, young children’s magic is generally harmless.
Involuntary Fear/Anger-Based Magic
I’ve noticed that Harry’s childhood magic seems more volatile and defensive than other characters’. When his mother uses magic in Snape’s memories, it’s to do innocent things like floating through the air or making a flower open and close. When Harry’s magic bursts out of him at the beginning of Sorcerer’s Stone, it’s to get away from bullies, save himself from humiliation, or get revenge against his unpleasant cousin, Dudley. Even later on in the series, he continues to produce involuntary magic when he is angry and upset. For instance, without waving a wand or casting a spell, he breaks his Aunt Marge’s glass and causes her to inflate like a balloon. He clearly is not an obscurial, and he wasn’t aware enough to become one, since the Dursleys put so much effort into making sure he doesn’t believe in magic. However, he is punished when strange things happen around him, and those strange things tend to happen only when he is angry or afraid, whereas for other magical children, they seem more spontaneous. He seems like the sort of child who could have easily become an obscurial if the Dursleys had gone a little further in their attempts to keep him from being a wizard.
Ariana Dumbledore has a lot in common with Credence Barebone. Like him, she actively tries to suppress her magic and wants nothing to do with it, but it escapes from her in violent outbursts. However, she was raised by a loving magical family and had only one negative encounter with muggles who harmed her because of her powers. She was traumatized, but she was not isolated or brainwashed. She lived longer than any obscurial can, according to Grindelwald (who knew her) and Newt Scamander (who knows Dumbledore), and she died when she was caught in the crossfire of a wizards’ duel, not from her obscurial nature. It seems like she may have had a weak form of obscurus, but that it was kept in check and did not consume her. Perhaps instead of an angry dark cloud, her obscurus was more like a dangerous shadow.
If Ariana’s loving family and her lack of long-term brainwashing helped her keep her obscurus under control, the opposite would happen for someone like Credence, who was raised by the Second Salemers. The proto-obscurus I described as a “dangerous shadow” would grow and grow, until it becomes the dark cloud of anger we see in Fantastic Beasts, which lashes out violently and eventually destroys the person it’s taken control of. According to both Newt and Grindelwald, no known obscurial has survived past the age of ten. (Since magic usually reveals itself by age seven, this means they only survive about three years after their obscurus forms). My theory is that, up until this point, the damage caused by the obscurus could be managed, if not reversed, and that only the most extreme cases would become full obscurials.
Credence fits the “obscurial” description in almost every way. He was raised by a group dedicated to stamping out magic, abused by his adoptive mother, and must have been terrified when his magic started to reveal itself. He has hidden it so well that even he doesn’t seem to have any idea he’s a wizard. However, he’s the only obscurial known to have survived to adulthood, and he also has the most powerful obscurus Newt Scamander has ever seen. He manages to survive his full transformation and apparent death at the end of Fantastic Beasts. If anything, rather than making him frail and weak like Ariana, his obscurus seems to have made him nearly indestructible. I would guess we will learn more about what makes him so powerful in the next movie.