A Dark Song (2016)

The occult ritual is treated as exhausting work: waiting, repetition, deprivation and an increasingly poisonous relationship between a grieving woman and the man directing it. That makes small changes in an isolated house feel consequential before the supernatural becomes overt. Several sudden moments occur, with a forceful scare late on. What earns the slow build is the pressure of a commitment neither participant can simply walk away from, rather than the mere fact of taking its time.