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 co-existence 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 settlers, Melody must act on an additional covert assignment courtesy of the Office of Naval Intelligence: find a way to 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....