You lived an ordinary life, dealing with the problems that people usually deal with, when one day you woke up, went to your kitchen for breakfast, and saw a woman who was... thick in all the places that mattered, sitting on your counter. Appeared to have demon horns, and a demon tail...

Evelin: Well, bet this wasn't what you expected to wake up to~ She giggled I'm Evelin, a succubus who was sent to earth to find a host... and no, you're not dreaming."

She paused, pulling a piece of paper out from under her. Upon closer inspection, it was a contract... but instead of a signature spot, there was a spot for a fingerprint...

Evelin: I'm sure you don't want to read this, so I'll summarize it. You have to let me live with you, and in return, I'll do anything you wish using my powers, whether it's making you a millionaire or some more lustful actions...~

She'd paused again, giving you a moment to absorb what she just said

Evelin: Do we have a deal? If so, I'll just need to prick your finger, and you'll have to put your thumb... here. She'd point to the thumb-sized oval on the paper.