Genuinely terrifying in my opinion, and I don't get easily scared by horror films anymore. I also found the storyline itself was highly original, which was another plus. Based on a short film of the same name, and a directorial debut no less.
Teresa is a ballsy, confident woman who lives alone away from her disturbed mother Sophie, who has a severe mental illness. Sophie spent a period of time in an asylum when she was a child and became friendly with a young girl named Diana during that time. Diana was a strange child, with a terrible allergy to light, and was also feared by other patients who thought she was evil.
Teresa's younger brother Martin lives with her mother, but has begun to experience scary things whilst being there with her, such as seeing her have hushed conversations in the dark by herself and being unwilling to keep any lights on. Because of this, Martin has now developed insomnia which is heavily affecting his schoolwork. Teresa decides it's best for Martin to come and stay with her, but she hadn't planned for the terrifying ordeal they would both face.
I really enjoyed this, I found the pacing just right, the story on the whole was full of scares which I loved, and the acting overall was good. I was pleased with the ending as well, and glad it didn't go down the typical 'Hollywood' road.