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.
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 Brotherhood, used small quantities of aconite as a medicine for his heart condition.
Pierre Bellec obtained aconite from a corrupt apothecary, threatening his life in the process. He used one dosage to poison the Mentor of the French Brotherhood, Mirabeau, and planned to use a second dose in order to eliminate remaining members of the Council, in order to rebuild the Brotherhood according to his own fanatical vision, reasoning that Levantine Assassins, Italian Assassins, and Colonial Assassins rose to greatness after similar tragedies.