Zarathos is a fictional character in the Marvel Comics series Ghost Rider. He is a demonic being who tortures and devours souls. He first appears in Marvel Spotlight #5 (August 1972).
As explained in Ghost Rider #77 (1973 series), the physical, stone body of the demon Zarathos had laid inert and inanimate beneath the earth for eons until his spirit was summoned by a Native American tribal sorcerer named K'Nutu to help his tribe. Zarathos was then offered a steady supply of souls to consume in exchange for his aid in vanquishing enemies of the sorcerer's tribe, and for offering his might in their service. This alliance proves mutually beneficial, flourishes and a strong cult builds up around Zarathos, which gets the attention of Mephisto, who decides that Zarathos has too much of a following and is depriving him and the other lords of hell of souls. Mephisto eventually confronts Zarathos and defeats him (by the deceptive use of his soulless pawn, Centurious).
Zarathos is then forced to serve Mephisto for many centuries in the Netherworld, an extra-dimensional land of the dead sometimes called "Hell", though still powerful enough to avoid total confinement. Mephisto sends his unruly vassal back to Earth on occasion, binding him to human hosts and allowing him to ravage souls (on a smaller scale). In modern times Zarathos is bound to Johnny Blaze; the combined Blaze/Zarathos entity manifests as a leather-clad flaming skeleton known as the Ghost Rider, ultimately serving the side of "good" while still using hellish means which earns the Ghost Rider a reputation of fear, and Johnny Blaze a never-ending sense of guilt. As time progresses, Zarathos would occasionally from time to time assert his control over Johnny Blaze's body. Zarathos was temporarily separated from John Blaze by the Crimson Mage. Zarathos gained additional control over the Ghost Rider, becoming more violent. As the Ghost Rider, he re-encountered Centurious. John Blaze had grown stronger, causing an eventual clash over Blaze's physical body, as Mephisto temporarily frees Zarathos to compete with John Blaze for his freedom. Before it can reach that point, Centurious (Mephisto's champion who defeated Zarathos thousands of years before) returns and has his pawn Sin-Eater use his Soul Crystal to suck Blaze's soul from his body. This greatly weakens Zarathos who, without Blaze's soul, finds himself dying, for as a demon, and thus truly not of the earth as mortals are, Zarathos cannot maintain himself upon the earthly plane unless he possesses the physical body of a mortal. Using the last of his strength, Zarathos splits the crystal in half, freeing not only Blaze's soul, but innumerable souls that had been consumed in the past. Before the crystal merges back into one piece, it sucks Centurious inside as well. Zarathos feels cheated out of his revenge against Centurious, but with the help of the now-dying Sin-Eater, the demon manages to transfer his essence into the crystal. Johnny Blaze is now free from the curse that Mephisto placed on him many years before and the first existence of the Ghost Rider comes to an end.
Near the climax of the events known as Secret Wars II, the Beyonder, angered at humanity and the entire plane of existence, travels to Hell and informs Mephisto of his plan to erase all existence contained in this universe. Mephisto pleads with the Beyonder not to commit such an act, going so far as to make a bet with him. The Beyonder agrees, under these terms: He chooses both a champion to represent him and a separate champion to represent Mephisto; if Mephisto wins, the universe will be given a twenty-four hour reprieve from destruction, but if the Beyonder wins, not only will he erase the universe from existence, he will give his champion ample time to exact revenge on Mephisto himself. The Beyonder chooses Spider-Man as Mephisto's champion and decides that Zarathos will be his own champion. Zarathos' goal is not only to corrupt Spider-Man's spirit and morals, but to prevent Spider-Man from stopping an assassination attempt on the Kingpin. Spider-Man overcomes Zarathos' nightmarish attempts at corrupting him and saves the Kingpin's life. Thus the Beyonder (after re-imprisoning Zarathos inside the Soul Crystal) allows the universe one more day to exist (during that additional day came the Beyonder's defeat and seeming death).
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Official Site: Marvel Comics
Links: Zarathos Wikipedia page Marvel Database Zarathos page KnightMare's Zarathos page