The twelve labors

In the twelfth labor and most difficult one, Herakles was ordered to bring from Hades (lower world) the dog Kerberos (Cerberus), a creature with three heads, a serpent for a tail and snakes all over his body, which was guarding the entrance to Hades. 
Herakles went to Eleusis and met with the priest Eumolpus, who initiated him into the Eleusinian Mysteries, the religious celebrations in honor of goddess Demeter and her daughter Persephone.
After that accompanied by Athena and Hermes, he descended into Hades at the promontory of Taenarum, in Laconia. There, he set Theseus and Ascalaphus free from their torments and took permission from the god of the underworld, Pluto, to take kerberos to the upper world, provided that he would use only his hands.
Herakles encountered Kerberos near the gate of Achaeron, one of the five rivers of Hades and by the mere strength of his arms seized him and carried him to Eurystheus, who immediately sent him back to Hades.

Herakles brings Kerberos (Cerberus) to king Eurystheus,
who is hiding in a jar, hydria 520 BC. 

Herakles is trying to tie with the chain Kerberos, while
Athena is watching behind him, amphora 520 BC. Louvre