This is book number 6 in the Halo series.
In the first, desperate days of the Human-Covenant War, the UNSC has enacted the Cole Protocol to safeguard Earth and its Inner Colonies from discovery by a merciless alien foe. Many are called upon to rid the universe of lingering navigation data that would reveal the location of Earth. Among them is Navy Lieutenant Jacob Keyes. Thrust back into action after being sidelined, Keyes is saddled with a top secret mission by ONI. One that will take him deep behind enemy lines, to a corner of the universe where nothing is as it seems.
Out beyond the Outer Colonies lies the planet Hesiod, a gas giant surrounded by a vast asteroid belt. As the Covenant continues to glass the human occupied planets near Hesiod, many of the survivors, helped by a stronghold of human Insurrectionists, are fleeing to the asteroid belt for refuge. They have transformed the tumbling satellites into a tenuous, yet ingenious, settlement known as the Rubble--and have come face-to-face with a Covenant settlement of Kig-Yar . . . yet somehow survived.
News of this unlikely treaty has spread to the warring sides. Luckily for the UNSC, this uneasy alliance is in the path of the Spartan Gray Team, a three-man renegade squad whose simple task is to wreak havoc from behind enemy lines in any way they see fit. But the Prophets have also sent their best---an ambitious and ruthless Elite, whose quest for nobility and rank is matched only by his brutality . . . and who will do anything to secure his Ascendancy and walk the Path.