The god of the underworld in Roman mythology, known for his three-headed dog, is Pluto (or Hades in Greek mythology). Pluto oversees the realm of the dead, ensuring the balance between life and death. His three-headed dog, Cerberus, acts as a guardian to prevent the souls of the deceased from escaping. Pluto's narrative emphasizes themes of death, afterlife, and the inevitable nature of mortality in human existence.