Chapter 1 can be found here (I've never posted a multichapter fanfiction on a wiki before; I'm just going to link the previous posts with the chapters in the new chapters.) Also on AO3.
Grindelwald spoke to two of his devoted acolytes, Carrow and MacDuff, just outside the borders of Nurmengard. A good man and woman, a witch and wizard who could be counted on to do whatever he asked without questioning or flinching, who devoutly believed in the greater good.
"Carrow, MacDuff," Grindelwald spoke, his voice deceptively mild yet firm. "You know what is expected of you. Go to New York and find Modesty Barebone, the younger sister of our dear Obscurial. I want her brought to me unharmed." They all knew that the Obscurial had been raised by Muggles - how else would one become an Obscurial, they reasoned - but only Grindelwald, his Legilimens, and now these two knew of the Muggle girl who was so dear to him that he had thought of her as a sister.
His gaze lingered on Carrow, their unspoken understanding echoing between them. While her ruthlessness often served him well, having the girl dead or severely injured would be counterintuitive to his plans. At this point, at least.
"Frightened nearly out of her wits, though," he added, "I would not object."
Carrow smiled wickedly, and he nodded in approval.
MacDuff stood at attention, strong and steadfast. Yet a sense of uncertainty momentarily crossed his face, and he asked, "Two of us for one little Muggle girl? Surely one of us would be enough for a child under ten?"
"Indeed," Grindelwald agreed, "but remember that I ran into some....difficulties the last time I was in New York. Their Magical Congress may hinder your objective."
MacDuff nodded in understanding. If MACUSA had spies on the girl, or even had her under their protection; if their Aurors had laid enchantments to detect any ally of Grindelwald's, if they were caught - this was why two were needed. One of them had to get the girl to Nurmengard. And if the other was caught? No doubt attempts would be made to free them, but sacrifices were often necessary in times such as these. If they were careful, though, they would not need to worry.
Grindelwald had already informed them both of their mission, in a private conversation away from prying eyes - especially the Legilimens and the Obscurial. Now, his final instructions clarified the gravity of their errand. They were both well aware of the task that awaited them, their purpose firmly etched in their minds. If they did not always seek to fulfill his wishes as though they were their own, well, then they would not be his acolytes, would they?
With a subtle nod, Grindelwald dismissed them. Carrow and MacDuff bowed respectfully and Apparated away, leaving behind only the lingering echo of their presence.
It mattered not to him how they got to New York - by ship or Floo or Apparating across islands - or how they would disguise themselves, if at all; how they would ensure that they were not taken into custody, as warrants were out for all his acolytes; or for how they would even find the girl in the first place and get her back here. He trusted his acolytes to do what needed to be done.
As soon as they returned, his grip on the Obscurial's wavering loyalty would be tightened, and the boy would do whatever he said. He only had to wait.
(Did I just look up Grindelwald's acolytes on the wiki page and pick a couple at random? Why, yes, I did.)