A Dark Song is a solid character-drama build around two people performing a (mostly) genuine magic ritual from an early-modern grimoire, the quality of the film is build on the interactions of the two ritualists and the complexity and rigor of the ritual being performed. For example, aspects like ritual purification, which means no sex and fasting, magical circles, elementals and classes of demons and other rigorous back-crushing antics are as far as I know genuine, though it's not a subject I have studied since my early 20's, because I grew up. Does this work as a horror film? No, not really, I never got into a horror mood when watching this film, overall it's more of a religious drama most of the time, though religious in an off-beat way, with some sleaze to the mix. They did pick the right actor to play the guy though, a fat, broad-headed dude with glasses and an alcohol addiction, in staid of playing with Star Wars he plays with wizardry, yes, I immediately believe it.