Complex sentence: The story of Jean Chouan is, therefore, almost certainly, in large part, legend, a legend that had been continuously nourished by a minority Catholic and royalist-legitimist faction that remains active until the present day.
Split into simpler sentences: The story of Jean Chouan is, therefore, almost certainly, in large part, legend. It is a legend that has been continuously nourished by a minority Catholic and royalist-legitimist faction that remains active until the present day.