Lily Collins introduces the third trailer for The Mortal Instruments: City of Bones, where she stars as Clary Fray, a young woman living in New York City who realizes she is one of the Shadowhunters, a group determined to rid the Earth of demons. Lena Headey co-stars as Clary's mother, whose disappearance leads the young girl to join forces with the Shadowhunters and take on the evil forces that lie within an alternate version of New York known as Downworld. Harald Zwart directs this adaptation of Cassandra Clare's popular novel series, arriving in theaters August 21.