One more labour remained—namely, to bring up the three-headed watch-dog, Cerberus, from the doors of Tartarus.  Mercury and Pallas both came to attend him, and led him alive among the shades, who all fled from him, except Medusa and one brave youth.  He gave them the blood of an ox to drink, and made his way to Pluto’s throne, where he asked leave to take Cerberus to the upper world with him.  Pluto said he might, if he could overcome Cerberus without weapons; and this he did, struggling with the dog, with no protection but the lion’s skin, and dragging him up to the light, where the foam that fell from the jaws of one of the three mouths produced the plant called aconite, or hellebore, which is dark and poisonous.  After showing the beast to Eurystheus, Hercules safely returned him to the underworld, and thus completed his twelve great labours.