Complex sentence: In his second conflict with the X-Men-allied group X-Factor, he tries to kill Multiple Man, who, in self-defense, creates a duplicate of himself within Mellencamp's body, supposedly killing him.
Split into simpler sentences: In his second conflict with the X-Men-allied group X-Factor, he tries to kill Multiple Man. In self-defense, Multiple Man creates a duplicate of himself within Mellencamp's body, supposedly killing him.