I have another idea of what power the main character could have, he could have the dark side of the force from, it has many abilities(my most favourite one is force lightning) if you want to find out more then search up star wars the dark side of the force.
maybe the mc could get the magical power the Ruler, basically its from the seven deadly sins, it has the power to invert all attacks sent to the user. If you want to find out more then go search up seven deadly sins the demon king.
If the mc going to lose his humanity will become a demon, if yes will he become a demon lord or a demon king or perhaps a demon god or maybe an great old one like Cthullu.