I, Warden (The Last Raptori Book 1)

Fourteen-year-old Warden-trainee Maels Raptori faces off against implacable alien invaders using powerful—and deadly—new technology to conquer Earth. Enslaved, spirited away to their homeworld, and alone in a society of violence, he needs all his wits and skills to fight back if he hopes to save Earth—and himself—from annihilation.