The mythical creature said to guard the entrance to the underworld in Greek myths is the Cerberus. This three-headed dog is often depicted as ferocious, ensuring that no souls escape while preventing the living from entering without permission. Cerberus serves as a symbol of the inevitability of death and the barrier between life and the afterlife. The creature appears in various myths, including the Twelve Labors of Heracles, where capturing him was one of the impossible tasks. Cerberus underscores the gravity of loss and the finality associated with death.