The Roman god associated with volcanic activity and the underworld, and linked with young Jupiter, was Vediovis, also spelled Vejovis.
Vediovis is an obscure deity whose surviving descriptions are fragmentary. Ancient writers and modern scholars connect him with volcanic eruptions, while Roman religious tradition also placed him among underworld-related gods. He was sometimes identified with a youthful form of Jupiter, creating a striking contrast with Jupiter’s mature public identity as Rome’s chief god.
Vediovis was portrayed as a young man carrying arrows or lightning bolts and accompanied by a goat. He had temples on the Capitoline Hill and Tiber Island, and his worship included festivals on January 1, March 7, and May 21. A spring sacrifice of goats was intended to avert disease.
The most common mix-up is with Vulcan, the better-known Roman god of fire and metalworking, or Pluto, the principal god of the underworld. Vediovis is a different, unusually obscure figure whose exact original function remains debated.