“The American Horror Story” superstar has joined the cast of  Mercy, an adaptation of a Stephen King story being  produced by Blumhouse and Wonderland Sound and Vision, THR is reporting.  The film will be distributed by Universal Studios.  Peter Cornwell is directing the horror thriller, which  revolves around two brothers (Chandler Riggs and Joel  Courtney) who go with their mother (Frances O’Connor)  to take care of their grandma. Unbeknownst to them, the grandmother is a witch  who made an evil deal using black magic years earlier.

 McDermott will play a man who long ago had a relationship with the mother and  still has feelings for her, even though he is now married.   Mercy is based on King’s short story Gramma, which was  first published in Weirdbook magazine in 1984.

Jason Blum and Wonderland’s McG and Mary Viola are producing. Matt  Greenberg wrote the script.   McDermott, who appeared in the well-regarded indieThe Perks of Being a  Wallflower, plays a key role on FX’s American Horror Story: Asylum after starring in the show’s first season.