The monster lore was so interesting! I wanna find out more.
I'm sorry if there are any mistakes in the speech, but I'm using a translator, so I hope you understand.
It was mentioned that he and the rest of your family members are disguised monsters. That's why you see those cracks. It's clear that he lost himself because he became obsessed with his heroism. Perhaps he couldn't control his disguise, and that crack and the black color are part of his body.