Heracles coming up from Hades with Cerberus. Detail, Roman sarcophagus.
The story of how Heracles managed to complete this labour is very complex and I will not go into all the details. It might be of interest however to know that this is the only labour expressly mentioned in the Homeric poems
“He [Eurystheus] once sent me here to fetch the hell-hound- for he did not think he could find anything harder for me than this, but I got the hound out of Hades and brought him to him, for Mercury and Minerva helped me.”
Od. Xi. 623 Translation by Samuel Butler
This has been seen as the crown Heracles achievements. It is told that he entered the underworld in Laconia with the help of Hermes and Athena. In Hades he accomplished several deeds no less impressive then the ones in the world of the living. He saved Theseus (an ancient king of Athens), he rolled the stone of Ascalaphus (the man who was to roll a stone over a hill, only to lose grip every time when he was almost over the top) and he even intended to fight Meleager and Medusa but was held back by Hermes.
The Hero found himself in the end facing Hades, asking about permission to bring Cerberus to the surface. The god allowed him to do so if he did it without the power of arms. It is said that the hero then simply dragged the hound to the surface. Cerberus unfortunately couldn’t stand the light and started spitting, thus creating a poisonous plant called aconitum.
Eurystehus was terrified when Heracles arrived with the beast and hid away in a pithos (large vessel) as he had done twice already. The hero then returned the animal to the underworld and thus finished the twelfth labour.