A determined young woman, who has recently lost her child (Susan Loughnane) and a damaged occultist (Steve Oram) risk their lives and souls to perform a dangerous ritual that will grant them whatever they want.
Folk horror is a particular subgenre that peaked around the '70s with the likes of The Wicker Man and The Witchfinder General, but has since come back into fashion with likes of Ben Wheatley's A Field In England and Robert Eggers' The Witch. A Dark Song, which is set in modern times but feels like it was originally written as a period piece, is along the same lines as other folk horrors and ends up with similar results.
From the very beginning, it's clear that A Dark Song is trying to creep you out with its layering of exposition from Steve Oram's occultist, talking about things like the Tree of Sephiroth and the ritual they're performing, the Abramelin. You've also got the old, creaky house in which they're planning to perform their ritual and the warnings from Oram's character to Loughnane's character and others.
Where the film falters isn't so much in the atmosphere, it's in the characters and motivations. You never get to a point where you believe the character's grief, and her constant questioning of the ritual and lack of belief doesn't really add up when you consider she's going along with the constant berating from Oram's character. Nevertheless, we're in it now - just like the characters.
For the most part, the screenplay works reasonably well and prefers to use atmosphere and some excellent music rather than jump scares, again in keeping with the tropes of subgenre. However, it all moves at a snail's pace, almost as if it's trying to wear the audience down like the characters. There's extended periods of silence, allowing the light and shadow of the old house to creep in. The only problem with all of that is, unfortunately, it runs the risk of losing you. That said, there are a few sequences - one, in particular, where she speaks to her son through a door - that are particularly well-executed.
Overall, A Dark Song is an interesting, if familiar, take on folk horror.