Basically it was about this kid who was an orphan iirc, or lived with his grandparents or something, can't remember, and one day he decides to do a demon summoning ritual. he manages to summon the devil, who happens to be a hot chick, and he asks her to marry him. They do marry or something and that beings their life as adventurers.
The devil chick was also extremely powerful, basically OP.

pic unrel