Owen Egerton’s Mercy Black is a Schrödinger’s Cat of a movie. Is it a realistic horror thriller about homicidal delusions, or is it a supernatural thriller about a malevolent specter? You’ll have to watch the film to find out, but you may not want to, since it’s not particularly good.
Daniella Pineda (Jurassic World: Fallen Kingdom) stars as Marina, a young woman who, fifteen years ago, when she was just a child, stabbed another little girl in the woods. She and her co-conspirator, allegedly, convinced themselves that a fictional spirit named “Mercy Black” demanded a sacrifice, and in exchange all their pain would be taken away.
In the years that followed, Mercy Black has become a viral phenomenon a la Slender Man, and Marina has been institutionalized under the care of Dr. Ward (Janeane Garofalo). As the movie begins Marina gets released under the care of her sister, Alice (Elle LaMont, Alita: Battle Angel), and struggles to adjust to “normal” life.
Source: IGN.com Netflix's Mercy Black Review