This classic of Satanic cinema was, personally, best known to me because Kate Bush's Hounds of Love sampled the "It's in the trees... It's coming!" line from it. So, let's be honest, I spent much of the film up until the scene where it appears, waiting for that: to save you the bother, it comes a medium visited by the hero mid-way through. It's a nicely-mundane depiction of black magic, with Karswell (MacGinnis) playing a Satanist whose abuses are in danger of being exposed by a group of scientists: to protect himself, he tags one with a runic spell which makes the victim the target of a summoned fire-demon. The death only brings more heat, led by American psychologist Dr. John Holden (Andrews) and Joanna (Cummins), the niece of his victim, so Karswell decides to apply the same technique to Holden. He's initially unconvinced this is anything more than a scare tactic, but when things start happening, exactly the same way as they did to his dead predecessor, it's probably a good time to re-evaluate that unshakeable faith in science.
Famously, he fought with the producer over how much should be shown of the monster; that's a bit of a mixed bag, better in long-shot than close-up, where time has not been kind to the effects. As mentioned, I liked the prosaic nature of things. Karswell is about as far from the stereotypical Satanist as you can imagine, running Halloween parties for the local kids, and not a threat - at least, until you cross him. His lack of anger is perhaps even more chilling, most notable as he calmly explains the situation, when Holden visits his country estate: "What is this twilight, this half world of the mind that you profess to know so much about? How can we differentiate between the powers of darkness and the powers of the mind?" However, the overall logic leaves something to be desired. It's clear the scientists are all ultra-sceptics, so it's not clear how any expose by them could hurt Karswell, and the irreversible nature of the threat means Karswell has no way to negate it, even when his first victim agrees to do anything. Generally, the way he behaves isn't exactly smart, more necessary to the plot. But as you'd expect from Tourneur, who is among the elite at such things, there's no shortage of atmosphere and this sustains things over the more wobbly elements.