Cerberus

Cerberus also known as the “hound of Hades” – was the multi-headed dog who guarded the gates of the Underworld, preventing the dead from leaving, and making sure that those who entered never left. A child of Typhon and Echidna, he was part of a monstrous family, which included Orthus, the Lernaean Hydra, and the Chimaera as well. Only on three occasions Cerberus was tricked by visitors of Hades: Heracles did it with his strength, Orpheus with his music, and the Sybil of Cumae with a honey-cake.

Family
According to Hesiod, Cerberus was the second of the four monstrous children of Typhon and Echidna, being born after Orthus, the two-headed hound who guarded the cattle of Geryon, but before the Lernaean Hydra and, quite possibly, the Chimaera – all of them multi-headed. Later authors list many other monsters among the siblings of Cerberus, including the Sphinx, the Nemean Lion, the Caucasian Eagle, the Crommyonian Sow, the Colchian Dragon, Ladon, and even Scylla and the mother of the Gorgons. Quite a family, indeed!

Portrayal
As to the number of Cerberus’ heads we have contradictory testimonies. Hesiod, for example – after pointing out that Cerberus’ monstrosity was almost indescribable – also adds that he, “the brazen-voiced hound of Hades,” is “fifty-headed, relentless and strong” creature who feeds on raw flesh. Pindar goes a step further, claiming that Cerberus has twice more, i.e., one hundred heads.
However, possibly due to practical reasons, in art he is almost exclusively shown with three heads, and sometimes even with two or just one. Later authors, trying to reconcile the descriptions, started claiming that Cerberus had three dog heads, the rest of them being the heads of snakes spurting from his back, with a venomous one also serving as his tail.
Heracles’ Twelfth Labor

The last – and, thus, most difficult – of the Twelve Labours of Heracles set for him by Eurystheus was bringing Cerberus up from the Underworld. Hades allowed this, but only on the condition that Heracles manages to do it without using any weapon.
Even though he was bitten by Cerberus’ serpent-tail, Heracles managed to put the dog in a stranglehold and persevere long enough until Cerberus finally passed out. After this, Heracles fettered him with adamant chains and dragged the hound to Eurystheus, who was so horrified by the sight that he instantly hid in his great jar.
Before leaving the Underworld, Heracles also managed to rescue Theseus, who had been bound to a Chair of Forgetfulness for daring to help Pirithous kidnap Persephone. The earth shook in dread when Heracles tried to raise Pirithous as well from his Chair, so he had no choice but to let go and leave him bound forevermore.
Other Myths: Orpheus and Aeneas
When Eurydice died, Orpheus went to seek his beloved in the Underworld. Cerberus wasn’t going to let him pass, but Orpheus charmed him with his music, and the hound, tamed beyond recognition, stepped aside.
When Aeneas visited the Underworld, he had some more than necessary help from the Sybil of Cumae, who threw Cerberus a honey-cake, spiced with few “drowsy essences.” Cerberus ate it and fell asleep in no time. The expression “a sop for Cerberus” originates in this story: it means quieting an uncooperative person by giving him a bribe.