Halo: The Flood — William C. Dietz

The Human-Covenant War, a desperate struggle for humankind's very survival, has reached its boiling point on the mysterious, ring-shaped world called Halo. But the fierce Covenant warriors, the mightiest alien military force known, are not the only peril lying in wait.

As the fortress world of Reach and its brave defenders were bombarded to rubble, a single cruiser fled the carnage with the battle's only human survivors--Captain
Keyes, his crew of a few hundred Marines, and the last remaining SPARTAN super-soldier, the Master Chief.

With the cruiser's artificial intelligence, Cortana, concealed in his battle armor, the Master Chief crash-lands on Halo in the midst of a massive Covenant occupation. Curiously, the alien soldiers appear to be searching for something hidden on the ring. Built by a long-dead race, Halo harbors many deadly secrets, but one overshadows them all. Now the Master Chief must lead the scattered troops in a brutal race to unravel Halo's darkest mystery -- and unleash its greatest source of power... .