The Magisterium Series is a planned 5-book middle grade reading level fantasy series co-authored by Holly Black and Cassandra Clare and is their first collaboration together. The series was acquired by Scholastic in April 2012. The first book, The Iron Trial, was published in September 2014, and hit #3 on the New York Times Middle Grade Bestseller List. The books are set in a world of dark magicians in which children train as apprentices to be warriors.