In Greek mythology, Cerberus (from Kerberos, demon of the pit), was the hound of Hades — a monstrous three-headed dog (sometimes said to have fifty or one-hundred heads), (sometimes) with a snake for a tail and innumerable snake heads on his back.
He guarded the gate to Hades (the Greek underworld) and ensured that the dead could not leave and the living could not enter. His brother was Orthrus.
Heracles' final labour was to capture Cerberus. First, Heracles went to Eleusis to be initiated into the Eleusinian mysteries. He did this to absolve himself of guilt for killing the centaurs and to learn how to enter and exit the underworld alive. He found the entrance to the underwold at Tanaerum. Athena and Hermes helped him through and back from Hades. Heracles asked Hades for permission to take Cerberus. Hades agreed as long as Heracles didn't harm him, though in some versions, Heracles shot Hades with an arrow. When Heracles dragged the dog out of Hades, he passed through the cavern Acherusia.
Orpheus used his musical skills to lull Cerberus to sleep.
