The story is based on a true event: in summer of 1497, Juan Borgia, Duke of Gandia, was murdered and his corpse was found in the Tiber. He was son of Pope Alexander VI., and member of the Borgia family who ruled Rome from 1492 to 1503.
But it is not certain that his brother Cesare Borgia was responsible for that murder. So my description is actually fictional.
It's known though, that Cesare Borgia had a friend named Michelotto/Miguel da Corella, who was an assassin, murdering people for his lord Cesare.