The final task given to Heracles was the most difficult: to go into the underworld and bring back the guard dog Cerberus, the three-headed hound of Hades (god of the underworld) who stopped people passing between the world of the living and the dead. No living person had ever returned from trips to the underworld, and so this task seemed likely to end with Heracles stuck in the world of the dead.
Before leaving for the underworld, Heracles needed to learn the secrets of passing in and out of the land of the dead. He visited Eleusinian to take part in a secret ceremony called the ‘Eleusinian Mysteries’, in which the godly secrets of passing through the underworld learnt by the goddess Demeter and Persephone (who Hades had captured, then released) were revealed.
At a place called Taenarum Heracles then made his way down into the underworld. He passed ghosts, monsters, and people who had been captured. Here he once again met Theseus, who had (along with his friend Pirithous) been captured by the Furies (demons with monsters in their hair) after they had tried to enter the underworld and steal Persephone from Hades, in order to be Pirithous’s wife. The two were stuck in chairs, with snakes that had turned to stone wrapped around them. Heracles ripped Theseus free (leaving some flesh on the chair), but when he tried to free Pirithous the whole world shook. Theseus was free to go, but Pirithous would spend forever in the underworld.
Heracles continued, and finally found Hades, to whom he asked if he could take Cerbus. Hades agreed, but only if Heracles could control the dog without using weapons or harming it. Heracles agreed, and on the banks of one of the five rivers that flowed into the underworld, he met Cerberus.
The dog did not go quietly, but Heracles used his great strength to control it, and eventually the dog gave up. Heracles carried it up to the living world, and back to King Eurystheus who, on seeing the hound that guarded the dead, ran and hid. Eurystheus finally agreed to let Heracles go, as long as the dog was returned.