Complex sentence: In fifth season, Adrian Chase is introduced as a district attorney in Star City who helps mayor Oliver Queen clean up the streets through the legal system, and secretly acts as a merciless vigilante who targets criminals and, sometimes, innocent people.
Split into simpler sentences: In fifth season, Adrian Chase is introduced as a district attorney in Star City who helps mayor Oliver Queen clean up the streets through the legal system. Simultaneously, Vigilante begins operating in Star City and mercilessly targets criminals and, sometimes, innocent people.