The twelfth labor of Heracles was to descend to the Underworld and bring the three-headed dog Cerberus to Eurystheus
Eurystheus was disappointed that Heracles successfully mastered all labors he had set, so the last thing he would ask from Heracles was something impossible, which violated the laws of nature:
To fetch Cerberus, the guarding dog of the Underworld, from his master Hades.
Cerberos was a frightful dog with three heads, the tail of a dragon and snakes all over his body. No one had ever managed to escape from this monster and the dark kingdom of Hades.
When he reached the point between the world of the living and the world of the dead, Heracles first glowered Charon, the ferryman of the Dead who grouchily passed the hero across the Styx and brought him to the Underworld.
There Heracles met a lot of souls, among them the Greek hero Theseus and Peirithoos, a man who was imprisoned in hell because he fell in love with the wife of Hades.
When they saw Heracles, they stretched their hands towards him, with the hope
that his strength would raise them back to life. Heracles took Theseus by the hand, but when he tried to do the same to Peirithoos, the earth shook and he had to let him go.
Then Heracles reached the Kings of the Underworld, where he humbly asked for the permission of Hades to give him his dog. Hades agreed under one condition: that Heracles would win a fight with the monster without weapons but only wearing a lion's skin.
Heracles grasped the head of Cerberus between his arms and strangled it . During the fight, the tail of Cerberus severely injured the hero,
but finally Heracles overpowered the beast and it surrendered.
After his victory, Heracles carried it off until Mycenae and showed it to the King Eurystheus . The mere sight of the monster terrified Eurystheus so much, that he ran and hid himself in a barrel.
So Heracles returned the beast to Hades.