The Collegium Chronicles, book 4: Redoubt
Mercedes Lackey2013

Mags, a young Herald trainee in Haven, the capital city of Valdemar, has very rare talents. Recognising this, the Kings Own Herald trains Mags as a spy, tasking him with uncovering the secrets of a mysterious new enemy who has taken an interest in Mags himself. Why is an even deeper mystery. The answers can only be found in the depths of Mags past, if he survives long enough to find them.