This writer has always felt that the job of a reviewer was not merely to whine and prattle about what does (or does not) happen on screen, but rather to try to introduce the film to the discerning viewer in a way which assists the decision as to whether to see it ... or not? It is projects like Dark Song, however, that make such a simple ambition seem ever so complex.
For starters, I will suggest that the best (and most recent) analog I can name is the excellent 2016 Autopsy of Jane Doe. Both films are clearly of the horror genre, both films work their magic by confining two strong personalities (and superb actors) in a confined space, and both films follow the Prime Directive of possibly every horror film ever made, namely "whatever can go wrong will go wrong."
But that's where the similarities end. Autopsy of Jane Doe is merely a solid and workmanlike exposition of a clever, entertaining, theme. Dark Song is simultaneously the debut of a writer/director with a distinct, clear and uncommon "voice"; and, at the same time, a horror tale so intricately involved with the minutiae, the moment-to-moment, of true ritual magic (is there any other kind?) that at times you almost, just for a split second, wonder if you are watching a reality show instead of a fictional drama?
No small feat, that. Especially from a country that can lay legitimate claim to having produced one of the only genuine, verified, ritual occultists in western history (Aleister Crowley). To watch a film like this aware that there are individuals who literally dedicate their lives to such practices takes the viewer to new levels of awareness ... and terror.
The end result is something about as distant from the dozens of currently popular "found footage" films ... as the earth is from the moon.
Plainly put, to appreciate this film, to drink in and savour every nuance, the viewer himself (or herself) must be willing to commit to the movie to the same extent that the central characters commit to the core ritual.
That is to say, lock yourself in, don't leave your seat, and, whatever happens, do not break the circle of salt.
If you do all that, you may not get to meet an "angel" willing to grant your special boon, but you will get a film experience you will likely never forget.
