Aconite is an herbaceous perennial plant. Its leaves and roots are extremely toxic. When Hercules was sent to Hades to face Cerberus, the beast's spit fell to the earth, and from it sprang the first aconite plant. Also, when Theseus came to King Aegeas (his father), Medea (the king's wife) tried to poison Theseus with an aconite-laced drink. When Aegeas saw the sword of Theseus as a mark of his paternity, he dashed the cup from Theseus' hands.
Picture: http://www.wildchicken.com/nature/garden/aconite_flower.jpg