Aconite
Aconite is a substance that can be medicinal in small doses for heart trouble, but can be used to kill in high quantities as an extremely effective poison.
History
By the onset of the French Revolution, aconite was infamous as a poison used especially by the Templars.
Hervé Quemar, a member of the Assassin Council of the French Assassins, uses small quantities of aconite as a medicine for his heart problems.
Pierre Bellec took a store of aconite from a shifty apothecary who did not care as much about dosages as other doctors, threatening his life in the process. He used the first dosage he had in order to poison French Mentor Honoré Gabriel Riqueti, comte de Mirabeau, and planned to use a second store in order to poison the remaining leadership of the Assassins in order to force them to rebuild themselves in a fanatical attempt to recreate the tragedies that befell the Levantine Assassins, Italian Assassins, and Colonial Assassins.