Complex sentence: Luthor is arrested, and Batman confronts him in prison, warning Luthor that he will always be watching him, who gloats that Superman's death has made the world vulnerable to powerful alien threats, and that "he" is coming to Earth.
Split into simpler sentences: Luthor is arrested, and Batman confronts him in prison, warning Luthor that he will always be watching him. Luthor gloats that Superman's death has made the world vulnerable to powerful alien threats, and that "he" is coming to Earth.