The Champion

In Taran Matharu's The Champion, the explosive final book in the Contender trilogy, Cade and his friends wage the ultimate battle for their lives—and the fate of Earth itself—in the Game set by the cruel and mysterious Pantheon...
Cade has managed to survive the duel with the Hydra Alpha—barely. But the Games are far from over.
By order of their cruel and mysterious overlord, Abaddon, Cade and his friends are sent off to war against the Greys, a humanoid race who have far surpassed humans in technology on their home planet. This attempt to move up the leaderboard, however, leads Cade to a game-changing revelation: The Pantheon—the millennia-old alien masterminds behind the Games—have a weakness.
With the right artifacts scavenged from the land of Acies and a heavy dose of courage and luck, Cade has exactly one chance to end their tyranny forever. But if he fails, the lives of his friends—and the survival of Earth itself—will be forfeit.