Vulthoom is a Great Old One introduced in The Sleeper of Ravermos. In the story, Vulthoom is the Martian Aihai's equivalent of Satan. Though most rational people believe him to be a myth, he is nonetheless greatly feared by the lower class. In truth he is a mysterious being from another universe, exiled by his fellows there and lying in wait on Mars in the underground city of Ravermos.
He plans to take over Mars and then conquer Earth as his next trophy. Because of his vast intellect and advanced technology, he seems godlike, but is in reality merely a very powerful alien who must rest for millennia at a time.
While under the influence of the hallucinogenic perfume of an alien blossom, one man envisioned Vulthoom as a gigantic, otherworldly plant, but the being's true form is unknown.