Complex sentence: Guan Ping slew Zhai Yuan and Wei's commanding officer, Cao Ren, realizing this was a planned maneuver, relinquished back to his castle, but found it to be occupied by Guan Yu who challenged him to a duel.
Split into simpler sentences: Guan Ping slew Zhai Yuan and Wei's commanding officer, Cao Ren, realized this was a planned maneuver. He relinquished back to his castle, but found it to be occupied by Guan Yu who challenged him to a duel.