The films of Osgood "Oz" Perkins stir that middle-of-the-night sense that something terrible might lurk just past the edges of your perception. The writer and director is adept at unsettling atmosphere (aided by scores from his brother, the musician Elvis Perkins) and at slow-burn horror entirely removed from the pat assumptions of genre moviemaking. You know how, in most ghost stories, once the character knows what the ghost wants, everything will be OK? Perkins' conception of the horrific is more interior and unfathomable: His characters (in the ghost tale I Am the Pretty Thing That Lives in the House, already streaming on Netflix, and in this new demon-possession thriller, actually completed first) never fully comprehend the forces that haunt them, even as they seem to know that those forces in some way reflect them.
The Blackcoat's Daughter finds two boarding-school girls (Kiernan Shipka and Lucy Boynton) forced to winter together while a malevolent force -- horned and fuzzy, in silhouette -- takes an interest in them. Meanwhile, a grown-up (Emma Roberts) road-trips back to that school, for reasons Perkins allows you plenty of time to puzzle over. It's all muted suggestion until, surprisingly, the third act turns bloody. Perkins' influences are more overtly displayed here than in I Am the Pretty Thing -- in look and sound design, a creeptastic motel room is pretty much Isabella Rossellini's apartment from Blue Velvet -- and its conclusion less satisfying. But few horror debuts unnerve and fascinate as much as this one.