You probably haven’t heard of Antrum, such is the dread and secrecy that surrounds this “cursed film”. And that’s for your own good. Antrum was made in the late 70s and implicated in the mysterious deaths of several film festival programmers and foolhardy horror fans. Now, though, some documentarians have dug up a copy at an estate sale and are sharing it with anyone brave enough to watch.

Or so goes the framing narrative of this part-found-footage-part-mockumentary horror. It would be wonderful to stumble across Antrum with little prior knowledge, in a FrightFest-style festival screening, perhaps, or as a midnight movie in your local cinema: remember back when The Blair Witch Project was still a “true story”? Sadly, the pandemic is denying many of us that pleasure, so we might as well openly appreciate the meticulous care that Antrum’s Canadian co-directors have put into fabricating the look of a 40-year-old, low-budget Bulgarian curio, utilising effects both technical and psychological.

The premise of the film-within-the-film is fairly straightforward: a brother and sister trek into the woods to the spot where Satan fell to Earth and dig a hole to hell. Lucy Rayner’s plummy narration adds an authoritative air to the “documentary” sections, in which film experts and occultists speculate on the effect that “subsonic frequencies” and sigils scratched on to 35mm film might have on the viewer.

Even knowing and appreciating the artifice doesn’t entirely deprive the film of its eerie power. At one point a “legal notice” appears on screen, giving audience members time to exit, if unwilling to accept the risk of “injury, mortal danger or death”. During that unnerving thirty seconds, you may have to reassure yourself it’s all just a very clever hoax… or is it?

• Antrum is in cinemas from 23 October and on digital formats from 26 October.