The movie is an interpretation of M R James's gothic short story Casting The Runes, and is generally considered by fans of the horror genre to be a cult British classic. Fans of M R James on the other hand appear to prefer alternative adaptations of his work, such as the many made-for-TV BBC productions.
While I'm a big fan of M R James I still absolutely love this movie, probably because I saw it first as a teenager, and was intrigued then by the central conceit of the story. Karswell, the dark magician, can summon a demon to murder his victim, but only if the victim unknowingly accepts from Karswell a runic curse carried on a piece of parchment. The climax to the movie, which turns on this maguffin, is one of the best of its kind.
There are many memorable scenes in this film, aside from those where the demon makes an appearance, although these too are genuinely spooky, particularly the accompanying sound and visual effects. Look out for the seance scene, and the scene where Karswell conjures up a storm at the children's party. Fans of Kate Bush may also recognise the cry "It's in the trees" as a sample on her Hounds of Love album.
The villain in M R James' original story was allegedly based on an infamous occultist of the day, and it was rumoured that James wrote the story as a means of defaming this particular individual (whose name escapes me - my apologies). While totally fanciful in the cold light of day, the possibility that the Karswell character may have once existed edges more into the corners of your mind as you watch this film on a cold, winter's night. Enjoy.