In the events of the Critical Mass storyline, when John Constantine visited Hell, he was confronted by the demon Buer, working to try an restore the First of the Fallen, bargained with John for his soul in exchange for all the children in Hell, including Astra Logue's, the girl he damned to Hell.
Not knowing what to do, The Phantom Stranger appears to John in a dream, and suggests he make a double of himself and send that to Hell instead. When asked why the Stranger is helping him he replies 'For balance.' John was able to conjure up a clone using powerful magic designed by Aleister Crowley which included putting part of Crowley's soul, John's Demon blood as well as all his guilt and negative emotions into the clone. The con worked and the souls of the children were released, though he was unable to prevent the First of the Fallen from being revived.
When the Constantine clone was sent to hell, Crowley took the punishment for himself, leaving the clone to wander Hell as a demon while remaining unable to touched by the other denizens of Hell.
Writer Paul Jenkins created the Demon copy of John in his 1995 issue of Hellblazer #95 during his debut "Critical Mass" arc. Jenkins wanted to symbolically address old plot threads he wanted to do away with by putting them into the demon duplicate and sending it away so he could start his run on Hellblazer with a fresh slate.
Major Story Arcs
After the events of Critical Mass, the Demon John made occasional appearances later in the title. At one point he attempted to merge back with John, although he eventually suggested John sleep with the demon Chantinelle to regain some demon blood.
During the final years of Hellblazer, the Demon John was accidentally summoned by John's band's old drummer Beano who was promptly killed by Demon John. He used his wedding invitation to attend the ceremony and kill John and marry Epiphany in his place and even sexually abuse John's neice Gemma (which he regretted). The Demon Nergal, wanted to destroy John's chance at happiness, crashed the wedding and ripped out his heart, not realizing the John he killed was the demon version. The real John was then able to sneak up behind Nergal and rip his spine out, after having been returned from the dead.
Although not the version from the comics, a reference to Demon John appears in the pilot episode of the NBC series Constantine titled "Non Est Asylum". When John confronts the electrical demon Fercifer on a rooftop, he changes form into a demonic version of John (while suggesting that it is his future). He summons an illusion of Astra to try and trick Constantine.