If you loved Separation, try Brahms: The Boy II
A bridge between a film you've already seen and one most people haven't. Here's what they share, and what the second one does that the first one doesn't.
What they share
Both films are directed by William Brent Bell, and they sit in Horror / Mystery territory. If that's the register that drew you to Separation, the second film will land in a comparable space — through a different lens.
What Brahms: The Boy II is
English countryside, summer, a porcelain doll's face. A family moves into a mansion, their son befriends Brahms, a doll with a disturbing presence. This sequel amplifies the eerie atmosphere of the first.

