This story is intertwined with events in SWAMP THING #76 and takes place during and after that issue.
Issue 10 of HELLBLAZER was finally reprinted in the 2007 trade paperback HELLBLAZER: THE DEVIL YOU KNOW, long after reprints of rest of Jamie Delano's run on the series. It is not included in the original (1992) HELLBLAZER: ORIGINAL SINS trade paperback. The HELLBLAZER: ORIGINAL SINS trade paperback reprinted issues 1 through 9, which is a little confusing because the story really concludes in issue 10. What happened to Zed and Nergal? Presumably, issue 10 was excluded from the trade paperback because it would cause even more confusion for people who did not read recent issues of SWAMP THING. The expanded 2011 edition of HB: ORIGINAL SINS includes ST #76-77, but still doesn't include HB #10. Hopefully this webpage will answer most of the questions for people who didn't get to read all of the books.
SUMMARY OF THE STORY THUS FAR: In his own series, John learned "There has been a prophecy incontrovertible - engraved on a stone dredged up from Hell. At the winter solstice, a conjunction between nature and super-nature -- a birth. The child will be a healing power in this realm." (Issue #8)
The demon Nergal and the Damnation Army want to fulfill the prophecy with the birth of a child that they can control. An opposing cult called the Resurrection Crusaders also wants to fulfill the prophecy to rule the world for "good". They plan to do this by offering one of their members, a old girlfriend of John's named Zed, to mate with an angel. John sees it as bad news for the world if either Nergal or the Crusaders succeed, and he plans to stop them somehow.
In HELLBLAZER #7, John is critically injured when he sleepwalks off a moving train during a nightmare. While unconscious, Swamp Thing takes him to the hospital in SWAMP THING #74. In HELLBLAZER #8, John is forced to make a deal with Nergal to be healed of his injuries by a transfusion of the demon's blood. In HELLBLAZER #9, John has sex with Zed, which makes her impure because his blood is tainted by Nergal (although Zed doesn't know it.)
Meanwhile, in the SWAMP THING series, a new elemental spirit had been prepared to enter the world to take Swamp Thing's place, while Swampy was lost in space and presumed dead (issues #55-63). The co-existence of Swamp Thing and this new elemental is causing havoc with reality. A sequence of deadly coincidences (referred to as a "synchronicity storm") is accelerating around Swamp Thing and his friends. The universe demands that Swamp Thing give up his life or destroy the unborn elemental spirit (referred to as the "Sprout").
Despite his great efforts, Constantine has been unsuccessful in enabling the Sprout to become the new Swamp Thing, which requires a human's death in fire.
At the end of HELLBLAZER #9, John comes up with a plan involving Swampy to resolve the Nergal/Crusaders problem, but at that moment, Swampy materializes in John's flat with his own plan.
Coincidentally (or due to the synchronicity storm), Swampy had come up with the same idea: Swamp Thing can use Constantine's body in order to get Abby (who has agreed to the plan) pregnant with a baby which would be the mortal form of the Sprout. In SWAMP THING #76, Swamp Thing forcibly takes possession of Constantine's body before giving John a chance to explain that he had the same plan. Swampy in John's body then goes home to make love with Abby, who is rather hesitant, but willing. They succeed in conceiving a child.
This issue of HELLBLAZER shows what Constantine's consciousness is doing during SWAMP THING #76 and finally John, Swampy and Abby all converge on pages 19-21, shown below.
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Page 1 A fetal version of Constantine flies through space as one of thousands of sperm cells.
Pages 2-3 He bursts naked from his sac in his adult body, high above London.
Pages 4-5 Now in a clothed intangible, astral form, he falls through his roof, through his flat, down past his neighbors Mighty Mouse and Mrs. McGuire. They cannot see him, but he can see their auras. John descends ghost-like through the basement and the sewer lines, unable to remember who he is.
Page 6 Sinking through the earth, past bones and strata, he remembers the name "Swamp Thing", as an underground train zooms into him.
Page 7 Gaining control of his astral body, he realizes that ST has forced him out of his physical body. John levitates up to the city street and (invisible to all, except a blind man's dog) bounces into the 8 Sussex Garden W2 Yoga Center. (Does anyone know if this place exists (or did exist) in real life?) Here he sees a class of people meditating in their own astral forms, but the yogi leading the class quietly tosses John out.
Page 8 John dreams up an "astral plane" (a WWI fighter plane) to travel to Glastonbury to observe Zed (the bald girl) and the Resurrection Crusaders.
Pages 9-10 Per their plan in recent issues of HELLBLAZER, the Crusaders give Zed to an angel to mate with and become mother to the prophesied child.
Pages 11-12 The angel rejects Zed, because John had sex with her in last issue, thus making her impure with his demon-tainted blood. The stormy wrath of God falls upon the Crusaders, killing them all.
Page 13 John, still hovering in his spirit-form (without the airplane), considers the carnage he has caused. He spots the astral form of a member of the Damnation Army spying on him.
Page 14 Nergal violently retrieves his spy and rejoices over the destruction of the "Tongues of Fire" Crusaders. All appears to be going as he planned, but...
Page 15 Nergal's captive "dryad", with hair made of long plant roots, is showing great distress. Swampy's mating with Abby is causing great turmoil in the Green. This informs Nergal that Constantine has tricked him by enabling Swampy to father the prophesied Child.
Page 16 Nergal rips open his own chest, releasing demon dogs to chase down Constantine's astral form.
Page 17 John flees into an underwater Atlantis-like dreamscape, where he encounters Nergal in the form of a whale.
Page 18 Constantine tries in vain to defeat the whale by dreaming up a harpoon. His only chance is to escape into the Green.
Pages 19-21 (See page scans above)
Page 22 John arrives back at his London flat, recalling the Newcastle incident...
Page 23 ...and finds the slaughtered bodies of Mighty Mouse and Mrs. McGuire.
Page 24 John knows who Nergal is now. They met long ago. John retrieves his file about Newcastle.
Comment: Cover artist Dave McKean also did the cover of SWAMP THING #77 and illustrated the BLACK ORCHID mini-series, written by Neil Gaiman.