Grindelwald sends Graves, either as his right hand man or his unwilling messenger post-MACUSA takeover, to one of his dark wizard allies to negotiate some terms/pick up an artifact/what have you. Grindelwald gets suspicious after Graves is gone for longer than strictly necessary.

He apparates to his ally's base and is furious to find an Imperius'd Graves. What happens next is up to you!

+ Grindelwald pretending to be calm, but slaughters the wizard and his followers when they least expect it
++ Conflicted Graves, if you're going the post-MACUSA takeover route