From Halopedia, the Halo wiki
Halo: Envoy is a novel set in the Halo universe by Tobias S. Buckell. It was released on April 25, 2017. The Halo: Fractures short story Oasis, also written by Buckell, served as a quasi-prequel to Envoy, introducing the novel's central setting and conflict.
Melody is dispatched as an envoy to the Sangheili Fleetmaster Rojka Kasaan who is a commander in the Joint Occupation Zone of planet Carrow. She comes to find that the ship master is in possession of the Spartans known as Gray Team. He had collected them (in secret) after the destruction of Glyke due to Operation: SUNSPEAR. Due to an uprising led by Thars, who opposes peace with humans, Rojka is distracted and Melody is able to sneak off to the storage room Gray Team is being held in. She manages to thaw the three Spartans from cryo sleep just before being rendered unconscious from wounds inflicted by Rojkas' guards.
Melody awakes to find herself in the presence of the infamous Gray Team. She realizes that Gray Team is somewhat in the dark about their current whereabouts and situation. They come to find that the Human-Covenant War has been over for more than six years and that they have been in cryo the whole time as prisoners to Rojka. Gray Team discovers they have left a dire situation stranded in space only to be brought back right in the middle of the fray of a new and desperate situation. They learn of the new JOZs and that humans and Sangheili are residence of the planet they are now seemingly stranded on.
Struggling to come to terms with their past actions concerning the activating of the NOVA Bomb, Gray Team now tussles among themselves as they try to grapple the reality of their new situation. While continuously being pursued by the vengeful Rojka, who in turn is also dealing with Thars's forces, Gray Team and Melody start trying to find their way through the Carrie desert toward a rendezvous previously set up with Melody. Gray Team and Melody finally make it to the pre-scheduled meeting location only to find an underground ONI facility buried in the mountains. They narrowly gain access to the base just before being destroyed by Thars and his followers. Soon Thars blasts his way into the base and forces Gray Team on the move again. At the same time, Rojka overhears the envoy attempt to warn Thars of a Jiralhanae chieftain's plot to destroy Carrow and rescues her to learn more about why the Jiralhanae arrived planetside. Despite his anger, he prioritizes the safety of his home over vengeance on Gray Team.
Out of the base and back out in the sand Gray Team and Melody make their way to the nearby human settlement which is currently in a battle. Once in town they discover there is more danger besides Thars. The Jiralhanae chieftain, Hekabe, supposedly under the authority of Thars, decides to set his plan in motion. He ignores Thars's orders and brings his ship and army to the surface of Carrow and begins his plot.
Gray Team and the local human government realize that Hekabe has started digging in the desert so they put plans in action to intercept him, shooting down his cruiser in the process. They team up with Rojka and his remaining forces to combat the new threat. Their plan to thwart Hekabe finds much resistance as the Jiralhanae continue burrowing deep into Carrow toward an ancient and forgotten terror, the mythical Sharquoi. Upon discovering his goal to try and control the Sharquoi, Melody, Gray Team, Rojka, and the human government enact a daring mission to intercept Hekabe. Their mission takes them to the edge of despair as they close in on Hekabe and find that they're too late. Hekabe, with the help of an ancient artifact known as the vertex, is able to control the mythical monsters and pit them against the humans. Aware of the danger Hekabe poses should he find a slipspace-capable ship, Rojka attempts in vain to convince Thars to scuttle his remaining destroyers, only for the Sharquoi to hijack the destroyers; Thars is subsequently executed by Rojka for endangering everyone. Gray Team and company challenge the enraged Jiralhanae and his thousands of Sharquoi in the depths of the Forerunner structure under the desert sand. They narrowly escape the destruction of the structure after the human governor gains control of the vertex, who summons the Sharquoi back to the Forerunner structure while also destroying the hijacked ships, sacrificing herself by burying them with a nuke.
Gray Team and Melody are taken back into UNSC custody once reinforcements are able to access the planet. Back at his keep, Rojka is stripped of his title as kaidon due to having lost all combat-worthy ships but is offered to be the Sangheili envoy to the humans due to his relatively extensive experience with them. Rojka accepts the position, recognizing he can bring about peace for both races. Melody is congratulated for securing Gray Team and acting as envoy and is then sent to be debriefed by ONI. Gray Team are debriefed and caught up on the six years they were in cryo. They learn that Operation SUNSPEAR was actually conducted after the peace treaty with the Sangheili and that the Covenant has been dismantled officially. Gray Team is then given a choice whether they want to be discharged from the UNSC or be put back into operation. They choose to stay active at the behest of a few requests, which CINCONI, Admiral Serin Osman a fellow Spartan-II, quickly agrees to.