Who was the god of the underworld in Roman mythology?

Orcus was a god of the underworld, punisher of broken oaths in Etruscan and Roman mythology. As with Hades, the name of the god was also used for the underworld itself. Eventually, he was conflated with Dīs Pater and Pluto.

A temple to Orcus may once have existed on the Palatine Hill in Rome. It is likely that he was transliterated from the Greek daemon Horkos, the personification of oaths and a son of Eris.

The origins of Orcus may have lain in Etruscan religion. The so-called "Tomb of Orcus", an Etruscan site at Tarquinia, is a misnomer, resulting from its first discoverers mistaking a hairy, bearded giant for Orcus; it actually depicts a Cyclops.

The Romans sometimes conflated Orcus with other gods such as Pluto, Hades, and Dis Pater, all gods of the underworld. The name "Orcus" seems to have been given to the malicious and punishing side of the ruler of the underworld, as the god who tormented evildoers in their afterlife.

Orcus was chiefly worshipped in rural areas; he had no official cult in the cities. This remoteness allowed for him to survive in the countryside long after the more prevalent gods had ceased to be worshipped. He survived as a folk figure into the Middle Ages, and aspects of his worship were transmuted into the wild man festivals held in rural parts of Europe through modern times