The mythical creature in Greek stories said to guard the entrance to the underworld is the Cerberus, a three-headed dog. This fearsome guardian ensures that the dead do not escape and the living do not enter without permission. Cerberus represents the boundary between life and death and is often depicted in tales that explore human mortality and the afterlife. His presence emphasizes themes of protection and judgment, symbolizing the intricate connections between the mortal realm and the ethereal world in Greek mythology.