Transformation into demon devil after barfly convinces you, soul, for sex

You have just picked up a smoking hot blonde from the bar and she is now back at your place. She says she will have sex with you and fuck your brains out, if you give her your soul. It seems like it is all in fun, but then she produces an actual contract almost out of thin air. As she is describing what will happen to you her skin starts to a transformation into a red color. The more she talks the more pronounced the red tone gets. She flexes her biceps and other muscles as she is obviously growing stronger and then, her voice suddenly becomes deep and ominous. You are in deep trouble.