“The Mortal Instruments” is the latest young adult fantasy book series to make the leap to the big screen, and the first film in the franchise is building an impressive cast of sic-fi and fantasy veterans. 

"Game of Thrones" star Lena Headey and Jared Harris ("Sherlock Holmes: A Game of Shadows") are the latest names to be added to “The Mortal Instruments: City of Bones."

They join Lily Collins ("Mirror Mirror"), "Twilight" vet Jamie Campbell Bower and "Lost's" Kevin Durand in the film. 

Godfrey Gao ("all About Women") is also coming aboard. 

“The Mortal Instruments” is a series of six books written by Cassandra Clare. In the first book, "City of Bones”, an NYC teen named Clary Fray (Collins) finds out she is the latest of a line of "Shadowhunters," a secret society of half-angel warriors caught up in an ancient battle against demons.

Harald Zwart ("The Karate Kid" remake) is directing from a screenplay by "Pretty Little Liars" scribes Jessica Postigo Paquette and I. Marlene King.

Headey will play Jocelyn Fray, while Harris will appear as Hodge Starkweather and Gao as Magnus Bane.

"Mortal" will open August 23, 2013.

For fans of the books, what do you think of the casting choices?