Stray Gods: The Roleplaying Musical

At the end of Act I you will be at the club, after your 1v1 song w Persephone you will go to her office w Apollo. They will have an argument/song that you can chime in on occasion, when that's over, choose to stay w Persephone. After the convo with her and the end of Act I the achievement should pop. (If it's your first playthrough you may have multiple achievements pop at the Act II screen.)