Complex sentence: Paul was the last of the popes appointed by the Medici (Florence's powerful ruling family); thereafter pressure from reforming monarchs in France and Spain, most especially from Charles IV, ended their hold on the papacy.
Split into simpler sentences: Paul was the last of the popes appointed by the powerful ruling Medici family of Florence. Thereafter pressure from reforming monarchs in France and Spain, most especially from Charles IV, ended their hold on the papacy.