Cerberus in Greek and Roman mythology, is a multi-headed (usually three-headed) dog, or "hellhound" which guards the gates of the Underworld, to prevent those who have crossed the river Styx from ever escaping. Cerberus is featured in many works of ancient Greek and Roman literature and in works of both ancient and modern art and architecture.
Mythology
Cerberus was the offspring of Echidna, a hybrid half-woman and half-serpent, and Typhon, a gigantic monster even the Greek gods feared. Its siblings are the Lernaean Hydra, a serpant woman; Orthus, a two-headed hellhound; and the Chimaera, a three-headed monster.[5] The common depiction of Cerberus in Greek mythology and art is as having three heads. In most works, the three heads each respectively see and represent the past, the present, and the future, while other sources suggest the heads represent birth, youth, and old age.[6] Each of Cerberus' heads is said to have an appetite only for live meat and thus allow only the spirits of the dead to freely enter the underworld, but allow none to leave.[7] Cerberus was always employed as Hades' loyal watchdog, and guarded the gates that granted access and exit to the underworld.
The Twelfth Labor of Heracles
Capturing Cerberus, without using weapons, was the final labour assigned to Heracles (Hercules) by King Eurystheus, in recompense for the killing of his own children by Megara after he was driven insane by Hera, and therefore was the most dangerous and difficult.
After having been given the task, Heracles went to Eleusis to be initiated in the Eleusinian Mysteries so he could learn how to enter and exit the underworld alive, and in passing absolve himself for killing centaurs. He found the entrance to the underworld at Tanaerum, and Athena and Hermes helped him to traverse the entrance in each direction. He passed Charon with Hestia's assistance and his own heavy and fierce frowning.
Whilst in the underworld, Heracles met Theseus and Pirithous. The two companions had been imprisoned by Hades for attempting to kidnap Persephone. One tradition tells of snakes coiling around their legs then turning into stone; another tells that Hades feigned hospitality and prepared a feast inviting them to sit. They unknowingly sat in chairs of forgetfulness and were permanently ensnared. When Heracles had pulled Theseus first from his chair, some of his thigh stuck to it (this explains the supposedly lean thighs of Athenians), but the earth shook at the attempt to liberate Pirithous, whose desire to have the wife of a god for himself was so insulting, he was doomed to stay behind.
Heracles found Hades and asked permission to bring Cerberus to the surface, to which Hades agreed if Heracles could overpower the beast without using weapons. Heracles was able to overpower Cerberus and proceeded to sling the beast over his back, dragging it out of the underworld through a cavern entrance in the Peloponnese and bringing it to Eurystheus. The king was so frightened of the beast, he jumped into a pithos, and asked Heracles to return it to the underworld in return for releasing him from his labours.