i like this idea
I agree with francisca.
If he hums say (insert gods name here (made up if necessary)) curses you for the blasfamous nature of your song.
If he converses and the other player talks back (as suggested several times) make them fail saves.
But if the other player doesn't respond...
Familiar or Unique?
You said that the pit fiend was the big baddie of the last game was he a common everyday pit fiend or something more unique? If in the case of the latter he can't become a pit fiend because shapechange states otherwise.