The Rubicon Protocol

2560. Humanity finds its back against the wall after the United Nations Space Command flagship Infinity dropped out of slipspace into a devastating ambush launched by the Banished. As this fierce enemy alliance seeks to claim a powerful weapon hidden within the ancient Forerunner ringworld known as Zeta Halo, the surviving UNSC forces now find themselves compromised and its leadership out of reach - with remaining personnel forced to abandon ship and take their chances on the fractured, unpredictable surface of the Halo ring.