'Mortal Instruments' Prequel Movie in Development (Exclusive)

Stephanie Sanditz has been hired to pen "Infernal Devices," an adaptation of Cassandra Clare's spin-off book series.

Young adult franchise hopeful The Mortal Instruments: City of Bones doesn’t open until August 23, but Constantin Films is already working on a prequel.

The company has thrown into development Infernal Devices, an adaptation of the spin-off book series written by Mortal Instruments author Cassandra Clare, and has hired Stephanie Sanditz to write the screenplay.

Mortal Instruments, a best-selling series of fantasy romance books, centers on a young girl who discovers she is part of a hidden race of angels known as Shadowhunters locked in a battle with warlocks and vampires called Downworlders.

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Infernal Devices is set in Victorian England and sees a new teen girl plunged into a journey of adventure and self-discovery. The books feature the ancestors of many Mortal Instruments characters as well as some character overlap.

Three books, titled Clockwork Angel, Clockwork Prince and Clockwork Princess, have been published.

Constantin is also on the hunt for a director to bring on Infernal Devices.

The company has a certain amount of faith in the power of Clare and her works. While not at the level of a Stephenie Meyer or Suzanne Collins, the author has a legion of devoted followers, mostly teen girls, the same demographic that has helped power other YA franchises.

As an example, Mortal Instruments held a panel at the recent WonderCon and Clare --  not actors such as Lily Collins -- had fans lining up to gush and ask questions.

Sanditz, repped by Gersh, is an actress-turned-screenwriter. Key in getting the Infernal Devices gig was her script for The Mediator, an adaptation of the ghost love story book series by Meg Cabot (The Princess Diaries) being developed by A Bigger Boat.