A director of psychology named Dr. Stewart nervously moves around his office. After writing his will, he commits suicide by jumping off the building's roof.In a state penitentiary, up-and-coming social psychologist Dr. James Martin arrives to interrogate a notorious serial killer and death-row inmate named Edward. Before meeting Edward, Martin reveals to the warden, Moss that he was brought in to fill in for Stewart, who was Edward's regular visitor for this procedure. Edward is scheduled to die via electrocution at his insistence that very day and Moss explains to Martin that Edward's fate depends upon Martin's diagnosis; if Martin deduces that Edward is sane, he will be executed but if he is declared insane via multi-personality disorder as Stewart had believed, Edward will be spared.Martin meets Edward who appears extremely self-confident and sure of things, knowing so much information about Martin and leaving him at a loss of words for an introduction. When Martin takes a seat to interrogate Edward, the inmate refuses to be a human and instead a demon inhabiting Edward's body named "Nefariamous," a name Satan gave him- "Nefarious" for short. Martin does not believe Nefarious is who he says he is and thinks instead that it is an act by Edward to seem insane, however he is surprised when Nefarious insists that he wants to be executed, or rather, that he wants Edward to be executed. Nefarious also explains that before the day is over, Martin will commit three murders.Martin, an atheist balks at this idea which annoys Nefarious who calls him a "pompous sack of meat" in believing that he is safe only because he thinks such things don't exist. Martin plays along, asking Nefarious how he came to inhabit Edward. Nefarious explains that he began tempting Edward from a young age to commit small sins before communicating with him through a Ouija board Edward received for Christmas. Nefarious possessed Edward's body only after Edward consented to the possession multiple times. Martin brings in a chaplain to confront the demon in Edward, which frightens Nefarious until he realizes that the priest does not believe in demons and therefore has no effect on him. Martin then asks to talk to Edward, causing nefarious to recede and produce a timid, childlike Edward who is terribly distraught and shaken by his possession. Edward insists that he is possessed and that the demon had murdered multiple people using his innocent body as a vessel. When Martin begins probing further, Nefarious takes control and insists that martin murdered his mother.Shocked that Nefarious knows this, Martin insists that his mother was dying of a terminal illness and was euthanize after a difficult decision on his behalf. When Nefarious infers that martin euthanize her to acquire her massive wealth, Martin becomes angry and threatens to leave the meeting but changes his mind. Nefarious explains he is mentally training Martin for an endeavor he wants Martin to do, and that he mentally tortured Stewart, driving him to take his own life so as to select Martin for this job. When Martin continues to be doubtful of Nefarious, the demon explains that he knows Martin's girlfriend is pregnant, and that because of Martin's threats to break up with her to avoid fatherhood, she is terminating her pregnancy.Nefarious explains that Martin can save his son with a simple phone call within the limited amount of time but martin refuses, thinking Nefarious is bluffing while also insisting that such a thing is his girlfriend's choice to make. However, overcome with curiosity, Martin begs a kindly prison guard for access to a phone, which he uses to call the Women's Health Clinic. Upon calling he realizes his girlfriend has just finished with the abortion. Nefarious cackles with glee, explaining to Martin that "All hell is rejoicing."A prison orderly asks Edward what he wants for a final meal before execution to which Edward gives specific directions for a simple burger, fries, and milkshake, only for Nefarious to assume control and tell the orderly to retract the order. Martin, now interested in what Nefarious has to tell him. Nefarious explains the war in Heaven from his point of view, saying that the demons rebelled in order to avoid an eternity of slavery, and that they have been prospering in the world. He also explains that he chose the death by electric chair because it was the most excruciating and painful death available and would be a "parting gift" before Edward's soul goes to hell for an eternity of more torment.Martin insists that the world is reclaiming the moral high ground by allowing all kinds of rights and freedoms, which Nefarious laughs at before revealing to him the truth about such "freedoms," including a decline in education and an influx of slavery and trafficking the demons help the elite manage behind closed doors. Martin begins pitying Edward/Nefarious and almost writes him off as insane to which Nefarious tells him to redact. martin is genuinely confused as to which Nefarious is, until Edward breaks through, begging Martin to help him. However, before he can tell Martin how to help him, Nefarious takes control again and punishes Edward by breaking one of his fingers before resetting it at Martin's insistence. Nefarious then explains the endeavor he wants Martin to do for him is to write a book called "The Dark Gospel" which is his Satanic equivalent to the Bible and will begin another world religion, this one in servitude to demons. Nefarious then lets Martin in on a closely guarded secret, that more people are currently going to hell than to Heaven.Martin is called away by Warden Moss who reveals to him that upon searching Edward's prison cell, they found a large documentation of Martin's life as Nefarious had done with all his other victims, as well as the dark gospel which Edward wrote, crediting it to Martin's name. Moss encourages Martin to condemn Edward, insisting that if he is spared, he will probably go after Martin. Martin, believing Edward is faking split personalities, confronts Nefarious about this. Nefarious breaks his hand to slip free of one of the cuffs and seizes Martin, threatening to break his neck unless Martin begs him for life. Overcome with terror, Martin begs Nefarious to spare him to which Nefarious complies before being beaten down by the prison guards. In the heat of the moment, Martin dictates that Edward is sane, but not before being warned by a triumphant Nefarious that he is condemning Edward to death.Edward's head is shaved and he is led to the electric chair. As Martin uneasily sits with the families of Nefarious' victims to witness the execution, he is complimented by the detective who spent 6 years pursuing Edward before capturing him. However, Martin is unsure of whether he made the right decision, remembering the "three murders" Nefarious told him he would commit which he surmises was his mother, his son, and now Edward. Before the execution, Edward pleads with martin to tell the truth about the demonic possession, but Martin is unsure what to believe and realizes he is too far ahead to do anything now. Before the switch is pulled, Nefarious takes control of Edward's body for a final time, asking if Martin accepts his offer, to which Martin silently refuses. Disappointed, Nefarious quotes, "Mene tekel upharsin," ("you have been measured in the scale and found wanting") before leaving Edward's body, causing Edward to suffer an extremely painful and terrible death in the chair.The demon then tells Martin, "You should have accepted my offer," before possessing him. Martin, under Nefarious' influence grabs the detective's gun and holds the witnesses at gunpoint. Martin then realizes that the three murders he would commit that day were instead of his son, Edward, and now himself. Nefarious forces Martin to commit suicide but martin inadvertently counteracts this by praying to God for help, causing the gun to jam. Nefarious lets go of Martin as he faints into the arms of the security.One year later, a visibly changed Martin is shown promoting Nefarious' book- not as a "dark gospel" but as a rewritten cautionary tale for people to read and realize the demonic influence in society. Despite insisting he is still an atheist, he reveals he has undergone many changes. On his way home, Martin sees a homeless woman rummaging through a dumpster. He gives her some money and she takes it before saying "Hello James," in Nefarious' voice, implying that she is Nefarious' new host. The screen cuts to black with Nefarious' hideous laughter overheard. IMDb