Cantarella

Silvio: "I will confer with my associates in the streets, but chances are good that I can procure some Cantarella."

Rodrigo: "Yes, and what is that?"

Silvio: "It is a most effective form of arsenic and difficult to trace."

—Marco Barbarigo and Rodrigo Borgia discuss Giovanni Mocenigo's assassination.

Cantarella was a poison, a variation of arsenic, often used by the Borgia family and other Templars who wished to rid themselves of unwanted people or enemies.

Pope Alexander IV and his daughter, Lucrezia Borgia, in particular used it in political assassinations.

Victims

• Doge Giovanni Mocenigo was poisoned by Carlo Grimaldi using cantarella.
• Pietro Rossi was poisoned by Micheletto Corella during the events of the Colosseum Passion Play with cantarella concealed in his wine; though Pietro was eventually saved by Ezio Auditore, who quickly brought him to a doctor.
• Rodrigo Borgia attempted to poison his own son, Cesare Borgia, with cantarella concealed within an apple. Cesare did not consume a lethal amount of the poison, but it did render him ill for months after.
• After Rodrigo Borgia attempted to poison his son, Cesare force-fed him the same cantarella-filled apple, resulting in his death.

Trivia

• It is interesting to note that in reality Cantarella was used to make whoever took it fall asleep for 4 hours. It appeared as if the person was dead, since they had no detectable pulse. Presumably with a higher, more concentrated dose it would prove fatal, as was the case with Rodrigo Borgia.
• When you infiltrate the Castel Sant'Angelo for the second time, you can hear Lucrezia say to a guard that she ordered Cantarella last night. The guard then says that Rodrigo had taken it. Presumably, the arsenic could have been for Caterina Sforza, but Rodrigo may have taken it in order to poison Cesare.