The poison is called coniine. In the Middle Ages it was used to relieve St. Vitus's Dance and epilepsy. Culpeper recommended the root to be used to treat gout. Qualified Homeopaths use it in tiny quantities to treat prostate problems. Unlike the Hemlock that off-ed Socrates, this nasty weed will cause a horrific death.
Death comes by paralysis and respiratory failure.
One of the big problems with this herb, is that it can easily be mistaken for Cow Parsnip, or wild carrot by the untrained herbalist. This herb is restricted in many parts of the world. Physicians use a form of its alkaloid hyoscine , in medications that help control Parkinson's Disease, motion sickness, and to relax a patient's muscle tension prior to surgery. It was once widely used as a sedative painkiller, but as I stated above, its use is now restricted. This ancient herb has a long history in Witchcraft.