The Story - Part 2
The hunt for Regret.
When the Prophet of Regret's assault on the city of New Mombasa is foiled by SPARTAN-117, the Covenant flee to yet another Halo ringworld—Installation 05, or Delta Halo.
In close pursuit, the Master Chief, Cortana, Sergeant Avery Johnson, and Commander Miranda Keyes divide their forces and land on this new structure. The Chief and Cortana pursue the Prophet of Regret across Halo's surface, while Keyes and Johnson search for the installation's library, the location of the activation Index.
But after witnessing the assassination of Regret at the hands of the Master Chief, cracks in the facade of the Covenant religion begin to show. The Prophets' trust in the Elites is quickly dissolved and they decide to turn to a different Covenant race for their security: the Brutes.
Without an opportunity to question this decision, the Arbiter is sent on yet another mission by the Prophet of Truth, this time to acquire the same Index the humans are after. When he arrives, however, he learns that the humans were the least of his problems. Under the command of the Prophets, the Brute Chieftain known as Tartarus betrays the Arbiter and leaves him for dead, seizing both of the humans and Delta Halo's Index.