Capturing Kerberos alive, 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. In the traditional version, Herakles would not have been required to capture Cerberus, however Eurystheus discounted the completion of two of 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, Herakles 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.