There was five main storylines in the run . The first , collected as " Original Sins " , deal with John travelling to America to exorcise a demon , Mnemoth , and investigate a strange cult known as Damnation 's Army , crossing paths with a demon called Nergal ( from whom he gain demon blood ) , and having to be responsible for killing an old friend , Gary Lester , and betraying another , called Zed , in the process . The following four issues , " The Devil You Know " finally explain John 's failure to save a young girl , Astra , from a demon in Newcastle , an event that left him near insane and incarcerated in an asylum known as Ravenscar , and still haunted him to the comic 's end . He eventually discover that the demon responsible for this was Nergal , and use a technological scheme to trap him , and lead him back to hell . It also contain a crossover with Swamp Thing , where Constantine lose his body while the Swamp Thing use it to procreate .