Cerberus is the fierce, multi-headed guard dog of Hades, the Underworld. He is often referred to as the "Hound of Hades".
Usually shown with three heads, though some sources list as many as 50, Cerberus was the vicious guard of the Underworld. Some sources describe him as having lion’s paws, a snake for a tail, and a lion’s mane of snakes’ heads, which may account for the 50 heads.
Cerberus is the offspring of Echidna, a hybrid half-woman and half-serpent, and Typhon, a gigantic monster even the Greek gods feared. Its siblings are the Lernaean Hydra; Orthrus, a two-headed hellhound; and the Chimera, a three-headed monster.
He guards the entrance to the Underworld, allowing the shades of the dead to enter and no one to leave. One bite from his venomous fangs was said to cause the dead to suffer agony for all eternity.
Orpheus lulls Cerberus to sleep with his lyre and singing, allowing him passage into the Underworld while still a living man, and the hero Heracles captured him as the last of his twelve great labors.
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"Cerberus." Wikipedia. Wikimedia Foundation, 6 June 2014. Web. 14 June 2014. http://en.wikipedia.org/wiki/Cerberus