Hemlock

Hemlock, more properly a member of the conium family of plants, is a deadly poisonous plant. It grows in the wild in damp areas. It contains at least four poisonous substances, the most important of which is coniine, which disrupts the function of the central nervous system.
Almost any dose of hemlock can be fatal as it directly affects the respiratory muscles, preventing them from working. The effect is progressive and, depending on the dose, can be quick or slow.
In Knight Fall, King Miles is found to have given hemlock to Sir William, but he innocently thought it was wild carrot.
Conium at Wikipedia
Category:Toxins