American Horror Story star Dylan McDermott is returning to the horror genre, signing on to star in the Stephen King adaptation Mercy.

The story, based on Stephen King's short story Gramma, follows two young brothers (Joel Courtney and Chandler Riggs) who discover that their grandmother is actually an evil witch. No details were given for Dylan McDermott's character. Frances O'Connor also stars as the boys' mother.

Peter Cornwell (The Haunting in Connecticut) is directing from an adapted screenplay by Matt Greenberg, with Jason Blum, Mary Viola, and McG producing.

It isn't known when principal photography may begin.