Cantarella

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

- Silvio Barbarigo speaking of the poison for Giovanni Mocenigo's assassination.

Cantarella was a poison, which was 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 for political assassinations.

Victims
Doge Giovanni Mocenigo, who was poisoned using cantarella by Carlo Grimaldi after the former refused to ally himself with the Templars.

Pietro Rossi was poisoned by Micheletto Corella during the events of the Colosseum Passion Play on the orders of Cesare Borgia because he was jealous of Pietro's relationship with Lucrezia. Micheletto concealed cantarella in his wine; though Pietro was eventually saved by Ezio Auditore da Firenze, who quickly brought him to a doctor.

Rodrigo Borgia attempted to poison his own son, Cesare Borgia, with cantarella concealed within an apple after Cesare abused his position as Captain General of the Papal army. 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 four hours. In this time, the person appeared to be dead, as they had no detectable pulse. Presumably, a higher, more concentrated dose would prove fatal.
 * When infiltrating the Castel Sant'Angelo for the second time, Lucrezia can be heard asking a guard for the cantarella she ordered the previous night.