The reason Grindelwald is so interested in controlling an obscurus is because he was overpowered by Ariana.
Ariana is an obscurus, and during the fight that ultimately lead to her death, she was able to overpower Grindelwald and take his wand from him, perhaps in her confused attempt to save Aberforth from the cruciatus curse. In the fight, it was Albus that killed her, which is why he was later able to defeat Grindelwald.
This timeline means that Grindelwald had the elder wand before going to Godric’s Hollow and meeting Albus. There would have been time because Aberforth says that he was “about to go back to school” after they had “known each other a few weeks.” This means that Grindelwald was doing something else at the beginning of the summer after leaving Durmstrang, researching and stealing the wand. He was already aware of the Hallows before meeting Albus so this is possible.
During the fight the elder wand’s allegiance shifted to Ariana, and then to Albus. This is why Albus took so long to fight Grindelwald, he knew that it would confirm that it was he who had killed Ariana. It did, and he lived with that for the rest of his life.
In other words, Ariana disarmed Grindelwald, who held the elder wand, and Albus killing her meant the elder wand’s allegiance was now his. This theory would explain Albus’ lifelong grief and guilt over Ariana’s death.
Dark. Sad. As honeybunchesofpwn notes in their response to this theory, “Considering how important the obscurus was to the first Fantastic Beasts movie, I imagine the sequels will flesh out exactly what you’re talking about. Makes me wonder how dark the movies will get if your theory comes true.”
So, unless a certain Mrs. Rowling confirms or denies, we’ll just have to wait and see and shed more tears over our beloved Albus.
