I didn't expect this movie to be so sad, Jesus Christ. That said, this is perhaps my favorite horror movie in a long time. Although, I don't think it's adequate to categorize it simply as one genre, because it's also as much of a drama and psychological thriller as it is a horror movie. If you're expecting a wild ride, jump scare type of film, this is not for you. This is a story about grief and trauma, with some tense, uncomfortable scenes and some visceral body horror interspersed throughout — and I loved it.
Kudos to Sally Perkins for doing such a great job portraying a desperate woman consumed by grief. Her character was both infuriating (for how manipulative and cruel she was) and tragically sad. She is so anguished by loss the loss of her child that she would do anything to bring her daughter back. Perkins aside, every actor there did an excellent job, from the possessed Ollie to the troubled teenager, Andy.
I don't want to say anything more lest I give too much away, but if you've thought about seeing this movie, and you enjoy horror films that are thoughtful and nuanced, then I think you should give it a go. It's streaming on HBO now.