I went ahead and watched A Dark Song tonight and I can confirm that the actual ritual is nothing like what’s depicted in the movie. (Spoilers ensue.)
There’s no magic circles (in fact the author specifically says that circles and talismans are false magic), no eating toxic mushrooms, no sleeping on the floor, no ritual shaving, no staring at a stone for two days, no drinking blood, no drawing runes all over the place, no speaking foreign languages (you’re specifically instructed to only speak in your native language), no whatever that was where she’s sitting in a pyramid and having cold water poured on her in her underwear, no drowning and revival, and it’s definitely not a two-person operation with a cynical alcoholic yelling at you. It’s mostly just praying and studying holy texts.
The magic squares aren’t part of the ritual, they’re tools you can use after you complete it. Even the robes Solomon is wearing are wrong. When he accidentally gets stabbed, that wound be grounds according to the book to abandon the process entirely and start over at a later date rather than carrying on and letting him die. His wish for invisibility is something that can supposedly be achieved with the magic squares, but I don’t interpret that part literally. There’s no spell in the book for death like Sophia wants to put on the people who killed her son, though there are spells for inflicting illness and one which the Mathers translation lists only as being evil and which should never be used. (Dehn’s translation says this is to inflict harm on a man’s genitals - Mathers was apparently suffering from Victorian prudissitude and decided to censor it.) You’re not summoning the angel to ask it for a favor, though - you’re summoning it because you want to establish permanent contact between yourself and it so it can serve as your mentor and constant advisor. Sophia doesn’t seem to even know what she’s getting into at the beginning - if I didn’t know that the movie was intentionally fictionalized, I’d think Solomon was just making stuff up to fuck with her and take her money (which he kinda is when he tricks her into providing jerkoff material for his fantasies).
They could’ve just made up a name for the ritual and it wouldn’t be necessary to change anything else - if anything, it looks more like something out of the Key of Solomon or one of the more goetic grimoires. (The Book of Abramelin is available as a mass-market paperback for twenty bucks, for chrissakes - you wouldn’t need a bunch of handwritten tomes in eldritch languages to carry it out.) With a few alterations and more gore and dark humor, it could even be an Evil Dead spinoff with the two of them performing a ritual from the Necronomicon. When the ritual goes off the rails in the third act and actual demons start manifesting it definitely gets feeling more like something Richard Matheson would have written. Shades of Hell House for sure.
The actual climax I thought was a little underwhelming after all that buildup. The costuming and makeup for the demons is interesting, but chopping off a finger with a bolt cutter isn’t that impressive for beings that ought to be able to psychologically destroy you. The angel does look something like what I imagine a Holy Guardian Angel would - a warrior of glowing light, large and intimidating, very “BE NOT AFRAID”, though I don’t expect it to look human in form.
That being said, it’s a pretty good movie. I love how the minimalist soundtrack keeps everything feeling tense and torturous. It’s really a story about coping with loss, exploitation, cabin fever, revenge, and forgiveness, using the ritual as a backdrop for a story about two people struggling with emotional demons more so than actual demons. I like the symbolism of our leads being named Sophia and Solomon - the former being the Gnostic incarnation of wisdom and the latter being the Biblical king apocryphally said to be a master sorcerer who commanded demons. It’s worth a watch.
If a quarter of the crazy stuff that happens in this movie actually happens to me I’d be deeply surprised. I start in 12 days.