This is the first chapter of a fanfiction I wrote (also on AO3). I haven't written the rest yet.
As an aside, for the purposes of this fanfiction there's only about a year in between the first and third Fantastic Beasts movies.
Questions and suggestions welcome!
The Obscurial was a problem.
He'd joined Grindelwald, yes, but not because he believed that wizards should rule over the world. Not because he believed that those of nonmagical blood were lesser. Not because he wanted to fight for a better world, a world where wizards ruled. Not for any of the reasons that his acolytes followed him. No, he'd only wanted to find out who he was.
Now that he did know who he was - or his real name, at least - his loyalty was wavering. His Legilimens had told him so; he regularly asked her to check up on his followers, but especially him. Her own loyalty was always suspect, of course, but all he had to do was drop a few remarks about how wizards and Muggles could live together in peace once he'd succeeded and about how no man, not even a Muggle, under his protection would come to any harm, and she was his again. But for the Obscurial, it wasn't that simple. Especially as he realized the consequences of his decision, as he saw what he was being asked - ordered - to do, what they were all training for. Foolish boy, war is war. Anything is necessary for the greater good.
Grindelwald wasn't about to just let him leave. He had put in immense time and effort to find the Obscurial, and it had been nearly as hard to get him to join his side.
He couldn't force him to stay. He was incredibly skilled, but the boy was an Obscurial - if he so chose, he could bring Nurmengard Castle tumbling down. He couldn't imprison him - his magic was the most powerful, destructive thing on earth. Above all, he could not risk the Obscurial turning against him.
The Obscurial was, however, emotionally vulnerable, and so far Grindelwald had used that in his favor. Deep inside, the boy wanted an authority figure to care about him, whom he could take orders from and trust unconditionally.
But if that stopped working? There was another way to get him to obey, a way that would ensure his loyalty without resorting to magical force. All because of the boy's foolish emotional sentimentality. But then, that was why he'd joined in the first place, so Grindelwald was grateful that the powerful Obscurial was so emotionally weak.
There was one person that Grindelwald knew that the Obscurial cared for. He'd even faced his worst fear, the thing he most dreaded, for her. The evidence from New York was more than enough, and his Legilimens had mentioned that he still thought of her.
If the Muggle girl Modesty Barebone was in danger, the Obscurial would do anything.