Aconite (also called Wolfsbane) is the deadliest poison known to man.
History[]
While under the influence of a Fomorroh, Merlin used aconite to poison Arthur's lunch. After he discovered that Gwen had already served Arthur, Merlin threw the poisoned food into a pigpen. It was eaten by one of the pigs, who died.
When Gwen saw the dead pig and what remained of the lunch, she took one of the chicken legs to Gaius, who determined that it had been poison with aconite (A Servant of Two Masters).