Complex sentence: In his second conflict with the X-men allied group X-Factor, he tried to kill Multiple Man, who in self-defense created a duplicate of himself within Mellancamp'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 Mellancamp's body, supposedly killing him.