Return of the Monster King

Our protagonist is named Ploúton Téras, a demon prince who is heir to the throne of monster kind. Before Ploúton's father; the pervious king, passed away. He quietly spoke to his child his last words. "My child, this world is not what it was used to be. As you take the throne, you will also realize the horrible truth of what our kind has become. Son, please grant me our kind greatest wish. I hope you are able to free monster kind from terror of humanity," After the death of the pervious king, Ploúton Téras was newly crowned king of the monsters. "Father, I swear on your name that I will grant your final wish,"