The film is based on Gramma (1984), a short story by Stephen King. The story mostly consists of an extended scene with young George forced to stay at home tending his grandmother. In flashbacks, we discover her history as a witch and deal with the entity Hastur, before George realises that Gramma has died but he then enters the room to find her corpse reaching out to him. The story reaches an ambiguous end where George appears to have inherited some of Grammas powers or even be possessed by her. The short story was previously adapted as an episode of the tv series The Twilight Zone (1985-9) with Barret Oliver as George. The film expands on the story a good deal, although oddly what we dont get is the scene with the dead grandmother that formed the centre point of the story.
Mercy is a low-expectation film but achieves its effect quite modestly. Gramma/Mercy is built up as someone the people around her are constantly suggesting is sinister in influence and power. The film gets great effect out of peripheral pieces Mark Duplass telling the story of the other sibling who wanted to leave home and get married but was found in the forest with the husband dead and she up a tree in fear. Or of how the young Mercy went into the mountains and came back pregnant. (Here King and the film willingly borrows from H.P. Lovecraft for whom Hastur was an entity, although Lovecraft had in turn borrowed this from an Ambrose Bierce story). This is definitely not a predictable film. You are held to it simply out of wanting to know what is going on. The climactic scenes with young Chandler Riggs trapped in the house as the force controlling Shirley Knight prevents him from leaving and wants to drag him into an occult circle as a sacrifice is undeniably creepy.
The film gets great usage out of Shirley Knight, an actress who achieved some fame in the 1960s, including two Academy Award nominations and a host of Emmy nods and wins in later years, but has steadily appeared in a series of supporting parts, usually as mothers and grandmothers since then. Peter Cornwell builds up her presence with fearsome regard, as much by people whispering about her in her absence that by her actual appearances on screen.
Other Stephen King genre adaptations include:- Carrie (1976), Salems Lot (1979), The Shining (1980), Christine (1983), Cujo (1983), The Dead Zone (1983), Children of the Corn (1984), Firestarter (1984), Cats Eye (1985), Silver Bullet (1985), The Running Man (1987), Pet Semetary (1989), Graveyard Shift (1990), It (tv mini-series, 1990), Misery (1990), a segment of Tales from the Darkside: The Movie (1990), Sometimes They Come Back (1991), The Lawnmower Man (1992), The Dark Half (1993), Needful Things (1993), The Tommyknockers (tv mini-series, 1993), The Stand (tv mini-series, 1994), The Langoliers (tv mini-series, 1995), The Mangler (1995), Thinner (1996), The Night Flier (1997), Quicksilver Highway (1997), The Shining (tv mini-series, 1997), Trucks (1997), Apt Pupil (1998), The Green Mile (1999), The Dead Zone (tv series, 2001-2), Hearts in Atlantis (2001), Carrie (tv mini-series, 2002), Dreamcatcher (2003), Riding the Bullet (2004), Salems Lot (tv mini-series, 2004), Secret Window (2004), Desperation (tv mini-series, 2006), Nightmares & Dreamscapes: From the Stories of Stephen King (tv mini-series, 2006), 1408 (2007), The Mist (2007), Children of the Corn (2009), Everythings Eventual (2009), the tv series Haven (2010-5), Bag of Bones (tv mini-series, 2011), Carrie (2013), Under the Dome (tv series, 2013-5), Big Driver (2014), A Good Marriage (2014), Cell (2016), 11.22.63 (tv mini-series, 2016), The Dark Tower (2017), Geralds Game (2017), It (2017), The Mist (tv series, 2017), Mr. Mercedes (tv series, 2017 ) and 1922 (2017). Stephen King had also written a number of original screen works with Creepshow (1982), Golden Years (tv mini-series, 1991), Sleepwalkers (1992), Storm of the Century (tv mini-series, 1999), Rose Red (tv mini-series, 2002) and the tv series Kingdom Hospital (2004), as well as adapted his own works with the screenplays for Cats Eye, Silver Bullet, The Stand, The Shining, Desperation, Children of the Corn 2009, A Good Marriage and Cell. King also directed one film with Maximum Overdrive (1986).
Jason Blum and his Blumhouse production company have produced a number of other genre films including:- Hamlet (2000), Paranormal Activity (2007) and sequels, Insidious (2010) and sequels, Tooth Fairy (2010), The Bay (2012), The Lords of Salem (2012), The River (tv series, 2012), Sinister (2012) and sequel, Dark Skies (2013), Oculus (2013), The Purge (2013) and sequels, the tv mini-series Ascension (2014), Creep (2014), Jessabelle (2014), Mockingbird (2014), Not Safe for Work (2014), Ouija (2014) and sequel, 13 Sins (2014), The Town That Dreaded Sundown (2014), Unfriended/Cybernatural (2014), Area 51 (2015), The Boy Next Door (2015), Curve (2015), The Gallows (2015), The Gift (2015), Jem and the Holograms (2015), The Lazarus Effect (2015), Martyrs (2015), Visions (2015), The Visit (2015), The Darkness (2016), Hush (2016), Incarnate (2016), The Veil (2016), Viral (2016), Amityville: The Awakening (2017), Get Out (2017), Happy Death Day (2017), The Keeping Hours (2017), Split (2017), Stephanie (2017) and Truth or Dare (2018).