Due to some absurd (and fun!) random tables my players got Demogorgon in a Bottle. Demogorgon has become his own character over the last few sessions constantly trying to make a deal with the party to let him out – which the party refuses. But they are caught – they dont want to let him out but nor do they want to just get rid of him just in case he falls into the wrong hands. One of my PCs (my wife) created Demogorgon in a bottle and now he sits in at each of our sessions:
I thought it was cool, especially since it happened organically.