Uncork’d Entertainment later acquired North American distribution rights to the film, later releasing it via Video-on-Demand and streaming services in Fall 2019.
Prior to the film, a concession stand worker dosed the popcorn with LSD; the combination of the drug and the imagery of Antrum resulted in a riot during which a pregnant woman was killed.
As the day progresses, Oralee is disturbed to find that her "spells" are having an apparent effect on the real world, conjuring actual infernal figures.
Additionally, the pair accidentally interrupt a man attempting seppuku, and obliviously pass by the rotting corpse of a suicide near their campsite.
Their first night in the woods, Nathan slips out of the tent and sees a boat being rowed in a nearby stream by a figure resembling Charon, who ferries a nude woman.
The next day, Nathan and Oralee stumble upon a pair of cannibals in the woods, who capture and cook people alive inside a giant, iron statue of Baphomet, including the man whose suicide attempt they earlier interrupted.
A "The End" title card appears onscreen; the film abruptly resumes, following Oralee as she runs through the woods, pursued by demons, and experiencing violent hallucinations.
As they would later recall in an interview with Rue Morgue, the central idea came from the concept of what it would be like to watch a purported "cursed" film with a history of harming the people who saw it.
Feeling that the idea would make an excellent horror film, the filmmakers began developing a screenplay based on the initial concept.
In an interview with Rue Morgue, both Amito and Laicini stated that the plot of the "cursed film" was deliberately made to appear as if it was "a dark fairy tale", with themes dealing with loss and moral ramifications of belief.
For Antrum's occult aspects, Amito, and Laicini studied various historical and cultural depictions of demons and the devil for inspiration, with their shared interests in religious imagery and the supernatural also factoring into the development of the film's script.
[3] American child actor Rowan Smyth, who had previously starred in the Christian drama film I Believe (2017),[5] was cast as Oralee's young brother Nathan.
[3] Principal photography for Antrum lasted for a period of one month in Southern California,[2] was shot under a low budget with only a handful of production staff working on the film,[2][3] with Amito later revealing in an interview with Rue Morgue: "We were wildly understaffed, wildly underfunded and unprepared for this month long shoot… yet, somehow a number of elements just magically landed in our lap."
Scenes involving Tompkins' and Smyth characters were filmed in a one-hundred acre, privately owned forest, where the filmmakers had been granted special permission to shoot at.
[12] It was later announced that Uncork’d Entertainment had acquired North American distribution rights to Antrum, and planned on releasing it via Video-on-Demand and streaming services in Fall 2019.
[18] Anya Stanley from Dread Central rated the film three out of five stars, writing, "Antrum is a multilayered indulgence of the imagination that uses both internal narrative and a mockumentary structure to blur the line between fiction and reality.
On his website, Kim Newman noted that, while the film was "technically ambitious" and commending some of its visuals, its thin plot, repetitiveness, and unconvincing documentary elements undermined it.