By the Gods!

Orcus

A god of the underworld in Etruscan and Roman mythology, Orcus was an intimidating fellow. He was known as the punisher of broken oaths, and while his appearance is never definitively agreed upon, he was almost certainly scary-lookin’, and is usually depicted as a hairy, bearded giant.  Orcus’ mother is Eris, the Greek goddess of Strife, also known as Discordia (her counter part was, quite fittingly, Harmonia). Sounds like the idea of pain/unpleasantness runs in the family, so it must not’ve been a surprise to Eris that her son turned out to be a bit of a tightwad/tyrannical daemon.

In many ways the Roman pantheon was transient, adopting different deities from different cultures (often freshly-conquered) and using syncretism to adopt new gods in an effort to pacify a newly-annexed population, and bring them under the Roman banner more peacefully. This went both ways, as some outside-gods became staples of Roman worship in certain areas. Chthonic deities (pertaining to the underworld) were often undergoing the switcheroo, but kept certain traits in common. Orcus has a great deal in common with the Roman Pluto, who in turn was partially derived from the Greek Hades. Dis Pater, an earlier chthonic deity, was subsumed by Pluto, though Pluto and Orcus seemed to feature in Roman culture simultaneously in different circles, and with different areas of focus. 

You’ve probably noted the “Orc” in “Orcus,” and you’re right on the money. “Orcus” was conflated with the aforementioned chthonic deities, but referred more specifically to the evil and punishing side of the underworld, which bridges nicely into daemons. His name came to be used for any number of daemons, and it is widely believed that the French “ogre” was derived from lo’ smelly-hell himself. J.R.R. Tolkien derived “Orc” from Anglo-Saxon, in which it meant simply “daemon,” for his fantasy works, though he personally believes that the root of this word does not lie with the Latin Orcus. Either way, from Tolkien’s “orc” is where we get the modern stock-fantasy-badguys that are so ubiquitous today.