Orcus is a Demon prince, one of the most powerful lords of the Abyss. He commands over the domain of undeath, although other Gods and demons are his rivals in this. Orcus is sometimes worshipped as a deity by those willing to cheat death by any means.
Having been stymied many times in his attempts to rule the Abyss (especially by the Demon Prince Demogorgon), Orcus now looks to expand to other realms. He often seeks footholds in Tol itself, but he is able to find many worshippers and servants in the Shadowfell. He seeks to seize power from one of the gods of death, notably Oldorin, for whom he has a special dislike. It may be that he has machinations to rise from his status as a Demon Prince to become a God himself.