Complex sentence: He is a city alderman who is running for mayor to create his vision of a Starling City, though leading a sinister double life as a criminal wearing a skull mask based from the nightmares he had of his late-father called "Brother Blood".
Split into simpler sentences: He is a city alderman who is running for mayor to create his vision of a Starling City. He leads a sinister double life as a criminal, wearing a skull mask based on the nightmares he had of his late-father, called "Brother Blood".