What three headed monster guards the gates of the underworld?

The three-headed monster that guards the gates of the underworld is Cerberus, a creature from Greek mythology. Known for his fierce demeanor and serpentine tail, Cerberus prevents the dead from leaving the underworld and ensures that the living do not enter without permission. He is often depicted as having three heads, each representing a different aspect of life and death. Cerberus is most famously associated with the myth of Heracles, who was tasked with capturing him as one of his twelve labors.