Cerberus looks like a black dog with three heads. In Greek mythology it is said to guard the underworld, land of the dead and the realm of Hades (the Greek god of death). Hercules (a Greek hero) was given 12 labours by the gods. The 12 labour was to capture Cerberus alive. Hercules accomplished this and then let Cerberus return to the underworld. In addition to guarding the underworld, Cerberus would prevent anyone from leaving it.