There's two settings for this.
Fantasy: You are a mage's/sorcerer's apprentice that seeks to surpass his/her master. The way to do this in your mind is summoning a powerful demon and making it your familiar. But something goes wrong. Either you miss a step or simply aren't strong enough, and the powerful demon you summon doesn't obey your commands. Not only that, but she decides you will be her new plaything and a warm hole for her cock.
Modern Fantasy: There's lots of reasons why you might summon a demon. You're bored, you're trying to prove a friend wrong, there's a need for revenge, or you're hoping it'll lead to an easy lay. But the spell is old, and the diagram for the demon trap is missing. Not only that, but the demon you summon isn't what you expect. She's bigger than you. Stronger than you. And she has a cock that puts yours to shame. But she won't leave. Not until she's fucked you.
Now, I see these either being short one shots or turning into a harem type situation where either your character wants to help the demon amass a group of men and/or women to use in order to establish her power on earth, or is forced/coerced into it with a combination of magic, mind control, and mind blowing sex. While I would like to play the demon in the picture, I would be more than happy to find a different one.