The real Percival Graves is freed from wherever Grindelwald locked him in.
Credence Barebone is stopped before anything worse could happen and thanks to some good words from the heroes of the movie is mostly left alone (although he still requires supervision).
Both are still hurting from the Grindelwald's actions.
Graves needs to feel like he's in control of his life again.
Credence just wants to feel warm, safe and loved.
They start drifting close towards each other for comfort from someone that knows what it feels like to be hurt. They stick together because they complete and balance each other. They stay that way because they fall in love.

All my love Credence calling him anything from sir to master to daddy (up to you to decide… hell, use all of them for all I care)
My everlasting gratitude for dirty talk. Bonus points for Graves telling Credence just how perfect he thinks he is while doing that (give me all the dirtiest compliments pls).
My eternal soul for tender and loving aftercare scenes