One film I feel keeps getting missed off the list is Roman Polanski's Macbeth. I suppose because it's Shakespeare, it's not thought of as horror nevermind folk horror. But Shakespeare can be horrifying and Polanski plays up that element.
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One of the reasons I don't think of Witchfinder General as folk horror is that the soundtrack is a pretty typical 60's cinematic orchestral fair. It's excellent, it's even very atmospheric but it doesn't have a relation between cinema and folk music. The film feels like it's from an older more classic era rather than the earthier feel of films from the 70's. This relation between film and score is firmly realised in the Wicker Man. What you see but also what you hear are very much part of the experience of Summer Isle itself, it's not just there to prime the mood.
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The soundtrack to The Blood on Satan's Claw is a definite move into something more genuinely unsettling. It's still an orchestral score but it's built around this very distinctive impish, piercing descending cromatic. A really simple but effective idea that changes the whole experience of the film. There's also avant garde noise passages, sliding strings, droning strings and a bit of theremin (or a theremin related instrument). It feels they're really pushing the boat out in creating an unnerving experience. It's something akin to then contemporary avant folk.
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Polanski uses the Third Ear Band on Macbeth who I would loosely describe as a drone based folk rock group. And they create something even harsher, less melodic and at times ear splitting than Mark Wilkinson's score for Blood on Satan's Claw. It's a full on horror soundtrack, that doesn't sound self consciously unnerving,
it just exudes terror and darkness in these harsh clashing tones which were simply part of TEB's musical vocabulary. It feels like a step beyond Blood on Satans Claw (I'm biased btw - I love TEB) albeit less memorable. Both films were released in 1971. But here there is a relation between the folk scene just as there is with the Wicker Man and Paul Giovanni and Magnet.
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Mark Kermode said Bobby Krlic's soundtrack to Midsommar reminded him of Paul Giovanni's for the Wicker Man. I really don't know what he was talking about. I definitely see parallels between it and the soundtrack to Macbeth though. Midsommar is tonely anti-horror - bad things happen but they're not presented as horrifying, it's all about cult indoctrination and the empathy and support that can provide (for a time) while quietly informing you that the cult istelf is both murderous and fascistic. Consequently the soundtrack is full of warm orchestral music with swelling, earthy melodic bass. But it pulls out some
disturbing high register drones to match the dissonance of the film itself. I definitely hear the Third Ear Band in that and to a less extent Mark Wilkinson's score.
So musically I see this version of Macbeth as being not just part of the folk horror tradition but as being one of its foundational keystones.