I wrote this once to describe my philosophy on what movies/TV I like:
"Ravenous fan of dark, edgy movies & TV. They should glue you to the screen and stay in your dreams."
Martyrs is one of a handful that describes that perspective to the T, and it is the kind of horror movie that you should go in completely clueless about the plot to enjoy the many twists that you won't see coming.
A big caveat: this is "advanced" horror -- gut-wrenching, gory and very disturbing. Definitely not for everybody.