Cyanide

Cyanide is a highly toxic chemical used as a poison.

History
Dr. Pentangeli gave Piero de' Medici a vial of cyanide which he gave to the Duke of Florence, who had been wounded by an arrow in an assassination. The Duke drank it, believing it to be medicine, and died as a result. Piero then attempted to wipe the cyanide traces with his handkerchief. (Dragon Hunt)

Whilst preparing the Duke's death mask, Tomaso noticed cracks appearing in the plaster cast template. Leonardo da Vinci tested all the chemicals used to preserve the body before wondering whether it had been the arrow that killed him. After testing a list of highly toxic chemicals provided by Machiavelli, Leo found it to be cyanide that caused the reaction, realizing the Duke was murdered. Meanwhile, Scarpa took possession of the handkerchief with the cyanide on it. Realizing he was under suspicion, Piero made a plan, getting Scarpa to shoot Pentangeli with an arrow dipped in cyanide to convince Leo that the Milanese had been responsible for the poison. (The Mask of Death)