Complex sentence: By condition of the Emperor, the Dauphiny could never be united to France; when the King of France had no son, he would personally rule the Dauphiny separately, as Dauphin.
Split into simpler sentences: By condition of the Emperor, the Dauphiny could never be united to Franc. When the King of France had no son, he would personally rule the Dauphiny separately, as Dauphin.