Imagine a normal British film, or even a soap opera (it was shot on the old Brookside (TV show) set) with naturalistic lighting and family arguments, all very kitchen sink, then 15 minutes in a group of black-clad SAS/SWAT guys suddenly appear and people start dying mysteriously and bloodily.
It's well shot, very claustrophobic (even at the start) and the cast are very good as they have to be without a lot of FX etc. help them carry the movie.
The advertising sells it as much more of a monster movie that it really is. This is a film about small rooms and fear.