Cerberus

Cerberus is the three headed dog with a dragon tail which guards the entrance to the underworld. Allowing the dead to enter but, never leave. Fetching Cerberus was the last labor of Heracles.

Encyclopedia Mythica
In Greek mythology, the three-headed watchdog who guards the entrance to the lower world, the Hades. It is a child of the giant Typhon

and Echidna, a monstrous creature herself, being half woman and half snake.Originally, the dog was portrayed having fifty or hundred heads but was later pictured with only three heads (and sometimes with the tail of a serpent). Cerberus permitted new spirits to enter the realm of dead, but allowed none of them to leave. Only a few ever managed to sneak past the creature, among which Orpheus, who lulled it to sleep by playing his lyre, and Heracles, who brought it to the land of the living for a while (being the last of his Twelve Labors).In Roman mythology, the Trojan prince Aeneas and Psyche were able to pacify it with honey cake.

Gods and Monsters
His sole task is essentially be the "bouncer" of Hades. As the rules go, only the dead may enter the Underworld, and none may leave. 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 famed musician, sneaks into Hades by lulling the usually unstoppable Cerberus to sleep with his beautiful music. The second myth is when Hercules (Greek mythology), with the approval of Hades (Greek mythology), the god of the Underworld, gets Cerberus in a choke hold, knocks him out, and kidnaps him. He is eventually returned to his post where he remains to this day. Cerberus, Guardian of Hades Other than these myths, the three-headed hound of Hell is an unmatched force for anyone trying to get in or out of Hades without express permission. In vampire mythology, it is said that the souls of all vampires are held in a container (a coffin, appropriately), somewhere in Hades. According to the Vampire Origin Story, because of a deal made by the first vampire, if any vampire should ever return to Hades they can get in (because technically they are dead), but they can never leave again. Some believe that due to his unmatched dedication to the gods of ancient Greece, Cerberus was eventually released by Hades and was able to join the gods in a more human immortal form, thereafter being known by the name Naberius.