So when Grindelwald take Percival's identity he takes the man and instead of killing him he gives him to Newt to train into a loyal beast. Percival is ready to die and endure any torture before he would do such a thing.

Once they're alone Newt reveals that he's working for the Ministry undercover, working to gain Grindelwald's trust. At first Newt just keeps Percival in his suitcase but when Grindelwald asks about him Newt is forced to bring Percival out, wearing a thick collar and making him act like a dog. Percival is humiliated but Grindelwald is delighted and begins to trust Newt. Together Newt and Percival keep the ruse going but it is deeply humiliating for Percival and with only Newt to trust he begins to fall for the man.

When Grindelwald wants to see a beast fuck Percival Newt refuses, insisting he used Percival for other things and thus they have to have sex in front of Grindelwald, Newt taking the bottom role to spare Percival further shame/humiliation/pain.

This all keeps going on and on until Percival is confused with what it real and what is fake. All he trusts is Newt.

Up to Annon if Newt actually is a spy or if he lied to Percival, coaxing him to break without having to torture him, impressing Grindelwald.