Aconite (also known as wolfsbane) is the deadliest poison known to man. Merlin used this poison to try and kill Arthur while under the influence of the Fomorroh. Merlin took a bottle of Aconite and poisoned Arthur's lunch, only to discover that it had already been made for him by Guinevere. The angry Merlin threw the lunch in the pigpen where it was eaten by a pig, who later died. This was discovered by Guinevere, who then told Gaius about it, causing him to suspect that something had happened to Merlin (A Servant of Two Masters).