Who is Arsenius from Pope Joan and what is their importance?

Arsenius is the father of Anastasius, Arsenius is the head of a very old and very wealthy Roman family with genetic ties to Caesar. He forces his young son to witness an assassination, telling him to watch and learn what happens when one speaks out against the emperor. Extremely ambitious, Arsenius plots and schemes from his mansion in Rome to position his son, Anastasius, as Pope but is ultimately unsuccessful.