Late Night with the Devil

The film's prologue is framed as a documentary investigating an unexplained event that occurred on the night of Halloween 1977, during the live broadcast of a sixth-season episode of the successful variety late-night talk show Night Owls with Jack Delroy, which competes for ratings with The Tonight Show Starring Johnny Carson.
Through his celebrity connections, Night Owls host Jack Delroy, who is based in New York City, makes regular visits to "The Grove," an elite California camp for wealthy and powerful men. After his wife Madeleine dies of cancer, Night Owls halts its production. Jack ultimately returns and, in order to boost the show's low ratings, does a special occult-themed episode on Halloween. Special guests for the episode include self-proclaimed psychic and medium Christou, skeptic and former magician Carmichael Haig, parapsychologist author Dr June Ross-Mitchell, and June's latest subject, 13-year-old Lilly D'Abo who is purportedly possessed by a demonic spirit. Carmichael promises to give anyone a huge cheque if they can prove that supernatural events do exist.
During the broadcast, Christou experiences a premonition about someone named "Minnie", which Jack reveals was his private nickname for Madeleine. Christou becomes sick, projectile vomits a black liquid, and is rushed to the hospital. In the next segment, June introduces Lilly, the sole survivor of a mass suicide by a Satanic church and its leader Szandor D'Abo that worshipped Abraxas. Jack persuades Lilly to communicate with the entity she claims possesses her, which she refers to as 'Mr. Wriggles'. During a commercial break, the crew informs Jack that Christou has died from hemorrhaging in the ambulance.
During June's conjuring, Lilly becomes possessed and levitates in her chair. The demon notes having previously met Jack "under the tall trees", revealing that Jack and June are romantically involved. Lilly's possession then subsides and the show continues.
Carmichael challenges June by subjecting Jack's sidekick Gus McConnell to a hypnotism demonstration, which causes nearly everyone in the studio to see worms pouring out of him. When the production team rewinds the footage, it proves that the demonstration was merely a joint hallucination experienced by nearly everyone in the studio; however, the supernatural phenomena that occurred during June's conjuring is unaltered in the recorded playback. Jack is horrified when he notices Madeleine's ghost standing behind him in the footage, but Carmichael accuses him of orchestrating the events. Lilly becomes possessed again; her head splits open and begins to glow like lava. She then brutally kills Gus and June. Carmichael is finally convinced that what he sees is a true supernatural event and begs for life as well as offering Lilly the previously mentioned cheque only for him to be killed as well. Jack is suddenly transported to a nightmarish version of the show.
He relives moments from the show's past before it is revealed that he had a prior connection to the demon possessing Lilly. During a ceremony at The Grove, he made a pact with the Devil, sacrificing his wife's soul in exchange for fame and the success of Night Owls. Thus, he was indirectly responsible for Madeleine's cancer. Madeleine's ghost begs Jack to put her out of her misery, as the cancer is causing pain. Using the ritual Athame dagger from Lilly's former Satanic cult, he then stabs her to death; and the scene suddenly shifts to the now-empty studio. A horrified Jack realizes he has stabbed and killed Lilly on live television. He stands over the bodies of his dead guests as police sirens approach in the distance.