Complex sentence: The Borgia family were allegedly masters of the use of poison and thought the use of straight arsenic to be too obvious and crude; once they perfected their "cantarella", the symptoms normally seen in arsenic victims were much more subtle.
Split into simpler sentences: The Borgia family were allegedly masters of the use of poison and thought the use of straight arsenic to be too obvious and crude. Once they perfected their "cantarella", however, the symptoms normally seen in arsenic victims were much more subtle.