After killing his family in an insane rage, Herakles (also known as Hercules) received orders from the oracle at Delphi to perform 12 labors to purge his sin. These were assigned by Eurystheus, king of Mycenae.
The last labor was the most difficult. Eurystheus sent Herakles to bring back Cerberus, the three headed dog of the underworld. Herakles set out, following the steps of Theseus, Pirithous, and Orpheus.
Down he went, past the snake headed Furies, past Sisyphus, Tantalus, and the other great criminals, past his friend Theseus, and finally arriving before the dark thrones of Hades and Persephone. Hades granted permission, provided Herakles used no weapon.
Before departing, Herakles asked for the release of Theseus, who had participated in, but not planned the ill fated attempt to kidnap Persephone. Theseus witnessed the battle with Cerberus, saw Herakles grip two of the three throats in a vice-like grip, threw the beast over his shoulder, and returned to Eurystheus, expecting, no doubt, the king's panicked response.