I’m almost sure this is easy… 80s or 90s weird horror movie. There was this well-dressed guy who was either a demon, or devil, or sorcerer, or wielder of some dark power; I can’t remember his intentions, take over the world or whatever; at a given point, his assistant, an attractive young woman, starts answering back, and he transforms her into a massive blob, with animal features, if I’m not mistaken. Then he walks away and she reverts to her original form. Can’t remember if in the end he becomes a monster or not…