A side note before I even get started--I was delighted to see the old Orion Pictures intro before the movie. I'll be honest in saying I had no idea Orion was even back in business!
Eighty workers are trapped in an office building, where they hear the voice of an unknown person telling them the only way to be let out of the building is to follow his instruction to kill their co-workers. If they do not follow the instructions, the unseen enemy will begin killing the employees off. This movie borrows largely from the legendary Japanese film Battle Royale, but if you're going to lift your idea from a movie, you could do a lot worse than that one. There are a ton of familiar faces in this film, including Tony Gallagher Jr. (10 Cloverfield Lane, Hush), Tony Goldwyn (Friday the 13th Part VI, Ghost), John C. McGinley (Se7en, Scrubs), Sean Gunn (Gilmore Girls), David Dastmalchian (Ant-Man, The Dark Knight), Abraham Benrubi (Without A Paddle, Parker Lewis Can't Lose), and, of course, Michael Rooker (The Walking Dead, Tombstone). The death scenes start early, and the violence is relentless all the way to the bloody end.