While imprisoned, Grindelwald and Graves realized they were soulmates. Neither wanted the bond, so they mostly ignored it. Flash forward to post-movie, Grindelwald is growing in power and realizes that, if he dies, all his hard work will be undone! He needs an heir! So he adopts a baby. Only, he's much too busy to raise him/her. So Grindelwald decides to drop said baby off at Graves's apartment, because if he has to have a soulmate, at least said mate can make himself useful.

Graves isn't amused and has no idea what to do. Does he take the baby to an orphanage? Raise him/her? Give him/her to someone at work? Next time Grindelwald turns up to check in, Graves is ready with a tirade.