“Game of Thrones” star Lena Headey, Jared Harris of “Mad Men” and Godfrey Gao have joined the cast of “The Mortal Instruments” from Screen Gems and Constantin Films.

Previously announced cast include Kevin Durand, Robert Maillet, Lily Collins and Jamie Campbell Bower in the pic helmed by Harald Zwart.

Based on Cassandra Clare’s bestselling young adult book series, the story follows Clary Fray, a seemingly ordinary teenager who discovers she is the descendent of a demon-hunting clan. Robert Kulzer and Don Carmody will produce and Bob Shaye, Michael Lynne, Martin Moszkowicz will exec produce.

Headey will star as Jocelyn Fray, Harris as Hodge Starkweather and Gao as Magnus Bane.

Headey’s credits include “300” and “Dredd” and Harris was seen in “Sherlock Holmes: Game of Shadows.”