Originally, the being called Volthoom existed on Earth-Three as a Tibetan monk. He had in his possession a unique power ring and lantern of great power unrelated to the Green Lantern Corps. Both the ring and the lantern were claimed by an individual who used them for his own ill-gotten gains, joining the Crime Syndicate of America as the nefarious villain Power Ring. It is unknown what became of the monk.
After Earth-Three was erased from existence during the Crisis on Infinite Earths and the Antimatter Universe was established as a complete mirror opposite of the regular DC Universe, Volthoom was once again established as a spirit trapped inside a power ring. The ring was a mixed blessing, however, as the wearer (Power Ring) gained great power but was bound to the ring and subject to Volthoom. The spirit constantly plagued Power Ring's mind in a subversive manner in an effort to ultimately gain control of him. It was theorized (but never stated in-comic) that Volthoom was the antimatter counterpart of Starheart. Power Ring was unable to remove the ring without having another accept the curse, and passed it on to a successor by trickery. Likewise, this pattern manifested when the new Earth-3 was created by the machinations of Mr. Mind.
Now, it seems these versions are just counterparts of a nigh-all powerful sociopath imprisoned by the Guardians of the Universe after once being their soldier.
Who and what Volthoom is, or was, is still a mystery. Eons ago, he was the very first Lantern created and empowered by the Guardians of the Universe in order to protect life. Rather than just wielding the green light of willpower, Volthoom seems to be able to wield a mixture of all seven parts of the Emotional Spectrum. However, this appeared to backfire, as Volthoom revealed himself to be, in the Guardians own words, an emotional sadist who enjoyed playing with the souls of living things for his own amusement.
Trying to rectify their mistake, the Guardians sealed Volthoom away, and half of their tribe chose to spend eternity keeping him imprisoned while the other half would go on to make other attempts at policing the universe. Some eons later, the Guardians returned to retrieve Volthoom from the brethren in a megalomaniacal attempt to eradicate free will in all living beings in order to protect life. They began siphoning off his energy in order to create the Third Army, mindless beings with no will of their own anymore, thinking that by eradicating emotion but not killing individuals the universe will finally be in complete order. But they failed and Volthoom escaped and took back all of the power the Guardians had stolen from him.
When he learned that his power had been corrupted by the Guardians lack of emotion, Volthoom set out on a quest to purify his reality-warping powers by consuming the emotions of beings who represented the various aspects of the Emotional Spectrum like Carol Ferris and Kyle Rayner.
Volthoom makes his debut in issue 9 of Green Lanterns. At the end of issue 9 he approaches a guy named Frank Laminski who is in despair and anger since he didn't get the green lantern ring. Volthoom tells him that he's been monitoring him for a while and tells him about the phantom ring. Frank is a sturdy person and tells Volthoom he'll do whatever needs to be done.
Later on Frank and Volthoom find Guardian Rami on earth, he possesses the phantom ring. While Volthoom handles Rami the hard way, Frank puts on the ring. Volthoom tells Rami that Rami ruined his life, that he is a traitor. Rami and the other Guardians put a battery in his life and now he looks like a ghost. He feels hatred with deep indignation.
Now Volthoom is in Rami's body while Rami's ghost is in a glowing ball in the hands of Volthoom. The other Lanterns/Guardians don't know Rami is actually not Rami, in fact he's Volthoom. They trust 'Rami' and he may stay on earth as protector.