Cerberus is best known as the guardian to the gates of Hades (the Underworld). In Greek mythology, Cerberus is an enormous, semi-transparent (he grows more visible the closer you are to death or the Gate of the Underworld) pure-bred three headed Rottweiler with the tail of a snake.
He is one of the great Greek monsters born unto Typhon (Last and the Greatest of the Giant) and Echidna (Mother of all Monsters. Typhon was a winged giant bearing a tail of a snake who could control and command the storms before been defeated by Zeus. He was the son of Gaia, mother earth of the Greek myths. Echidna, on the other hand, is a half-woman-half-serpent monster in the Greek tradition, who had mothered all or most of the monsters appearing in the mythology.
His task is essentially being the "Enforcer" of the Underworld. As the rules go, only the dead may enter the Underworld, and none may leave. If living tries to enter the Underworld Cerberus will make a meal out of them.
Zeus, Poseidon, and Hades went to the Underworld to rescue the Elder Cyclopes and Hundred-Handed Giants. They came cross Charon, the Ferryman of the Underworld. They crossed the River Styx and on the other side was the Gates of the Underworld. Before they open the Gates, Cerberus appeared and ready to attack them. Zeus was to prepared to fight but Hades was able soothe the giant hound. For saving him, Hades was given Cerberus by Echidna to secure his control over the Underworld.
There are only a couple of myths in Greek mythology where a hero gets the better of Cerberus. The first is when Orpheus (Greek mythology the son of Apollo and Calliope, Eldest of the Muses), the famed musician whom music can enchant anything and anyone, sneaks into Hades by lulling the usually unstoppable Cerberus to sleep with his beautiful music. There are some other tales as well that tell us that Cerberus could be subdued and lulled by playing melodious music to him. These legends show that Cerberus, no matter how formidable he seemed, could be hushed and captured, though with great efforts.
The second myth is when Hercules (Heracles in Greek mythology), with the approval of Hades, the Ruler of the Underworld, gets Cerberus in a choke hold, knocks him out, and kidnaps him. If he could remove Cerberus from the Underworld, with only his brute strength and without killing him. In that myth, when he took him out of the living world, Eurystheus immediately told him (by messenger) that to complete that labor he had to return Cerberus.
He is the father of the hellhounds by Nyx. Hellhounds are feared by demigods (except by children of Hades). Hellhounds are very large, tall, and can shadow travel. Hellhounds are usually described as being large, black mastiffs with glowing red eyes. The average sizes of hellhounds are about the size of a grizzly bear or a rhino, but can be as large as a garbage truck.Their purpose is to hunt down escapee spirits and damned people to the Underworld. If you bartered your soul to a demon; be prepare to drag to hell.
In a way, Cerberus stands for the sharp line of distinction between the world of the mortals and the world of the dead. The myth of Cerberus attempts to highlight the fact that life and death are two distinct concepts which can never overlap with one another. If any such effort is made by any individual, he may have to face grievous consequences for the same. This also applies to the dead and their souls that reside in the underworld.
Meet the sons of Cerberus