Etymology
Cerberus is said to be the sibling of the Lernaean Hydra, the Nemean Lion, the Sphinx, the Ladon, and the Chimera.
"Cerberus" is generally pronounced in English with a soft C as in cell, even though the ancient pronunciation, in both Greek and Latin was with a hard C as in cat.
Cerberus was the offspring of Echidna, a hybrid half-woman and half-serpent, and Typhon, a fire-breathing giant whom even the Olympian gods feared. Its brother is Orthrus, always depicted as a two-headed hellhound.[10] The common depiction of Cerberus in Greek mythology and art is as having three heads, a mane of live serpents (similar to Medusa's hair) and a snake's tail. 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.[11] Each of Cerberus' heads is said to have an appetite only for live meat and thus allow the spirits of the dead to freely enter the underworld, but allow none to leave.[12] Cerberus was always employed as Hades' loyal watchdog, and guarded the gates that granted access and exit to the underworld (also called Hades).[13]
The Twelfth Labor of Heracles
the tasks as Heracles had received assistance.
After having been given the task, Heracles went to Eleusis to be initiated in the Eleusinian Mysteries so that 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 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, which Hades agreed to 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 that he jumped into a pithos, and asked Heracles to return it to the underworld in return for releasing him from his labors