The Evil Within (2017)

Dennis is a 30-year-old man with a disability who has had nightmares all his life since he was a child, including one where he and his mother entered a carnival ride that seemed underwhelming until he is told that "the ride isn't over." As an adult, Dennis has one where a living demon, called the Cadaver, fastens a zipper onto his back and enters his body. He finds a hidden room in the cellar that holds a mirror that Dennis believes the Cadaver came from. He begins speaking to his reflection, who seems to answer back to him.

Dennis' brother John has difficulty of his own. His girlfriend, Lydia, demands that they get married, but is upset that he will not do anything about Dennis. He is visited by Mildy Torres, a woman from social services, who believes that Dennis needs to be taken away. This upsets John as he feels that Dennis is his responsibility. Dennis continues to have nightmares of the Cadaver and his reflection informs him that in order to get rid of it, they need to perform animal sacrifice. He begins working in the secret cellar, which perplexes John, especially when he sees that Dennis has videos on taxidermy and embalmment.

Once Dennis begins killing children, his reflection takes on a more competent demeanor and reveals that "he" was the one giving him the nightmares and that they still have more killing to do. Dennis meets with his crush Susan, an employee at his favorite ice cream place, and tries to ask her out, but the conversation gets awkward and Susan turns him down, hurting him. He returns and scares her where she runs out into the street and gets killed by a moving vehicle. When John and Lydia take Dennis to his favorite restaurant, he supposedly kills a man in the bathroom without hesitation, scaring himself. Dennis attempts to destroy the mirror but his reflection pushes him in and takes on the "real" Dennis instead.

After a night out, John and Lydia discover that they cannot seem to recognize anyone, implying that Dennis has killed more people. Torres arrives at the house with two officers and are all killed by Dennis. John reveals to Lydia his secret: Dennis was a child prodigy whose works had been published in the newspapers. One day while arguing, John punched Dennis and he fell down the stairs. When he awoke, his mentality diminished greatly and John has felt guilty ever since. Lydia sympathizes with John and leaves to get Dennis. However, John's friend Pete reveals that the taxidermy and embalmment tapes are Dennis' and later calls the cops.

Lydia arrives at the house and is stabbed by Dennis, who proceeds to remove her insides. John arrives and enters the cellar, where he sits in a chair with a spotlight on it. Dennis appears with a now-stuffed Lydia and puts on a ventriloquist act and John realizes he is glued to the chair. The previous victims have all been stuffed and are used in an extravagant diorama revealing that John intentionally knocked Dennis down the stairs to get rid of him. As more stuffed bodies are revealed, it becomes unclear if John or Dennis is seeing what is actually happening. Overwhelmed, John takes a gun and shoots himself. Dennis is slammed down by a Cadaver-Spider hybrid only for it to be revealed that he is actually being restrained by police officers who drag him away.

Following his arrest and trial, Dennis ends up in the padded cell of a psychiatric hospital. The reflection Dennis is still in control while the real Dennis is trapped deep in his subconscious. All Dennis can do now is wait to have the dream where he can get off the carnival ride.