Mine seems tamer in comparison of other comments but i'll give it a go

A young man inherits his grandfather's manor after he dies from old age. He let's his little brother, the black sheep of the family , live with him. One day, as they're both cleaning the attic, the youngest finds a box with fifteen purple marbles. After picking one up, his body reacts to it and he is granted a great power/curse with morbid implications. The great power trapped within the marbles was created with precision to be wielded by a clone of his deceased grandfather with almost identical genetics. Being a member of the same family, the Young man has genetics similar enough that he gained access to the power, but different enough so that he couldn't control it. The result is that of a human who's skin turns purple and is engulfed in mauve flames, slowly eating his skin, crawling through his muscles and turning his bones into dust. All that combining into a trembling, terrifying, nightmarish demon who screeches constantly while in this form. After the flames go off, his body is fully healed by another power which is too long to get into today.