One main theme could be the struggle between good and evil. It often explores how the devil or devil - like characters interact with the world of the good. Another theme might be temptation, as the devil is often associated with leading people astray.
Often in filth manga, the settings and situations are complex. Casual speaking could break the atmosphere or not fit the overall mood. Also, it might not convey the intended message effectively.
One similar novel could be 'Good Omens'. It also involves elements of the supernatural and has a unique take on good and evil. Another might be 'The Dresden Files' series which has a lot of action and magic intertwined with a battle between good and bad forces.
It was uncertain whether the fairy tale was an original because there was not enough information to determine whether it came from a specific work or author. Comics are usually created by cartoonists based on their own creativity and inspiration. They can be based on existing stories, novels, movies, or other works, or they can be completely new storylines. Therefore, to determine whether a comic was original or not, one needed to look at the source and background of the comic to determine whether it was based on a particular work or author.
There was no specific recommendation information about demon king manga novels in these results. Therefore, I don't know the specific recommendations for demon king manga novels.