Left alone with his ailing grandmother when Mom takes brother Buddy to the hospital, George discovers that granny made a deal with a demon, and his soul is part of her bargain. That's not a summary for "Mercy." That's a summary for the Stephen King short story "Gramma," upon which "Mercy" is based. It's only one sentence because the tale is so straightforward that Harlan Ellison adapted it into a quick 21-minute episode of "The Twilight Zone" in 1986 starring just two visible actors.
In expanding the work into a feature length film, "Mercy" understandably needed to embellish the details beyond merely one boy and his grandmother. So "Mercy" added a grandfather who dies of a self-inflicted wound, a triplet birth for grandma, an expanded role for brother Buddy, an aunt in the loony bin, an uncle in the bottle, an imaginary friend for George, a love interest for mom, a "devil worshipping" artist wife for the love interest, a concerned local reverend, and some occult mythology about death wolves and weeping books.
In expanding the work into a feature length film, "Mercy" understandably needed to embellish the details beyond merely one boy and his grandmother. So "Mercy" added a grandfather who dies of a self-inflicted wound, a triplet birth for grandma, an expanded role for brother Buddy, an aunt in the loony bin, an uncle in the bottle, an imaginary friend for George, a love interest for mom, a "devil worshipping" artist wife for the love interest, a concerned local reverend, and some occult mythology about death wolves and weeping books.