Halo: Envoy - Buckell, Tobias S.

It has been six years since the end of the Covenant War . . . and yet on the planet Carrow, a world on the edge of the Joint Occupation Zone, a decisive new battle suddenly erupts. Human colonists and the alien Sangheili have already been living a tension-filled coexistence in this place, with Unified Earth Government envoy Melody Azikiwe attempting to broker a lasting peace between their two species. But as civil war now engulfs the Sangheili, Melody must act on an additional covert assignment courtesy of the Office of Naval Intelligence: free the Spartan-IIs known as Gray Team, held in stasis since the end of the war by a cunning Elite fleetmaster consumed with vengeance. And none can anticipate the ongoing violence leading to the discovery of an even greater, unstoppable threat one hidden for eons below the surface of the planet.