Young adult fiction is the latest craze in Hollywood and studios are doing everything they can to lock up as many properties as humanly possible that even hint at being successful with children. And now two actors from two very different, and popular, television shows, have been cast in the latest hot property.

Lena Headey from the violent and extremely popular 'Game of Thrones' and Jared Harris, who stars in 'Mad Men' and gave Sherlock Holmes a run for his money as Moriarty, have been cast in 'The Mortal Instruments: City of Bones' based on the young adult novel by Cassandra Clare. Canadian actor Godfrey Gao has also joined the cast.

According to the Hollywood Reporter, 'Mortal Instruments' tells the story of "a young woman who discovers she is the descendant of a secret cadre of half-angel warriors locked in an ancient battle to protect the world from demons." Lily Collins has been cast to play the lead from a script by Jessica Postigo and I. Marlene King.

No word yet on the characters Headey and Harris will be playing. Headey will next be seen in 'Dredd 3D' which screened at Comic-Con and got rave reviews from those in attendance for being a violent, bloody comic book thriller. Harris will be co-starring in Steven Spielberg's 'Lincoln,' which opens in theaters this December.