Intruders [Celluloid Hero]
There's a general rule about horror movies: everything is scarier when it involves children. An adult being haunted is scary, but a child being the subject seems to hit closer, reaching a deeper level and making the story that much creepier and more effective. "Intruders" doubles the impact by having two children suffer.
"Intruders" is set in both England and Spain, focusing on a young girl and a young boy who are both being harassed by the same supernatural being known as Hollowface. The creature seems to feed off the fear of the children, growing in strength every time their parents attempt to console them.
Clive Owen is one of my favorite contemporary actors, he has never disappointed me in anything I've seen him in. He plays the patriarch of the English family, determined to protect his daughter. He doesn't believe in the supernatural being, instead thinking it's a regular human. The daughter continues to be traumatized, and her physical reactions to the mental and emotional trauma become more evident.
I won't give away the ending...but I will tell you I had an inclination to how it would end a bit earlier than it's actually revealed. Still, the impact of the events hit me strong enough to leave an impression.