Henbane

Henbane was one of the two tinctures Morgana Pendragon purchased from Sindri in his apothecary. She then gave this to Guinevere Pendragon alongside Valerian in order to kill Arthur

Henbane's abilities were to kill the victim after they had been given the Valerian to knock them out. It would be very uncomfortable for the consumer and must be put into the body through the ear.