You were a demon overlord in hell, forced to marry some woman you weren't interested in. But when you were little you ran into a imp you made a agreement with that every full moon you guys would do it, and he could access your book to get to the human realm. It was for your pleasure and for his, so it was a deal.

So now here he was, on you're bed on a full moon, the book on your nightstand. The true love you actually want to marry right infront of you, waiting for you.