Sacrifices, the third book in the Shadow Grail series by the New York Timesand USA Today bestselling authors Mercedes Lackey and Rosemary Edghill.
The students of Oakhurst Academy believe they have triumphed over the Shadow Knights. But Spirit, Burke, Muirin, Loch, and Addie know better. Under the guise of a company called Breakthrough Adventure Systems, the Shadow Knights have actually taken over the campus. The new regime is brutal, designed to turn the students into soldiers wielding both weapons and magic. Anyone who protests disappears.
Desperate, the group decides that Muirin should go undercover to spy on Breakthrough. But Muirin's act is a little too good, and Spirit begins to fear that her friend's loyalties might have truly changed. Surrounded by enemies and friends who suddenly seem like strangers, Spirit has decide who can--and cannot--be trusted.