Please note that I have not read the entirety of any of the Halo books or comics, but my friend and I have an interesting theory for Halo 5: Guardians. The Halo 3 terminals feature messages sent between the Librarian and the Iso-Didact, which includes the Librarian talking about the Mantle of Responsibility. After playing Halo 4, I realized that many of these messages foreshadowed the events of Halo 4. I then started looking for clues that may have been foreshadowing Halo 5: Guardians. One of the first pieces I noticed was the Librarian’s direct use of the word, “Guardians,” which stands out highlighted in the terminal amongs the other words. She used Guardians when referring to the line the Forerunners had constructed. The line was basically a safety border. In the Ur-Didact’s alternate attempts to stop the flood (such as the Prometheans), he realized that it would be impossible to save those behind the line, for that area had already been entirely consumed by the Flood. The Librarian was behind that line, which was why the Didact put so much work into developing an alternate way to stop the flood, other than activating the Halo arrays.
Skip forward to the Halo 5 teasers, Spartan Locke refers to the Master Chief as a traitor. It appears that the game may take place on the planet Sangelios, homeworld to the elites, and if that is the case, the Flood may return. In the Halo Escalation comics, a tiny flood spore was shown on the Spirit of Fire ship, which was drifting towards a planet that looked like Sangelios. It’s said that even the to tiniest Flood spore could start an outbreak. Not only that, but 343’s new Hunt the Signal game revealed photos that resembled a Flood spore, further hinting towards the Flood’s return. Say that in Halo 5, there is an outbreak of the Flood on that planet. Master Chief may have had to become a “Guardian” and choose to get rid of the flood in a way that would wipe out the planet, sort of as if that planet was behind “the line.” Obviously Master Chief would never activate a Halo array (especially since the Librarian’s indexed seeds to repopulate humanity no longer exist), but that might not mean he doesn’t realize its too late to save that planet. Therefore, he makes a decision that brings havoc to that planet, but saves the rest of the galaxy. Agent Locke, not realizing why Master Chief did what he had to do, could easily refer to him as a traitor to the ones he was supposed to protect. This would make Master Chief one of the “Guardians” the Librarian was referring to.
Now, the next aspect is what choice the Master Chief made. Long ago, the Didact first tried to stop the Flood with Prometheans, which we learned in Halo 4. However, the Didact also tried one more additional solution before the Halo array was activated, as learned in Halo 3’s terminals. That solution was Mendicant Bias, a highly intelligent Foreunner AI occupying a colossal Forerunner robotic body. The Didact wanted to use Mendicant Bias to draw the Gravemind outside the line and kill him when he’s vulnerable. Unfortunately for the Didact, the Gravemind corrupted Mendicant Bias’s mind and turned him against the Forerunners. As a result, the AI of Mendicant Bias was fragmented while the body was buried beneath a sandy area. The fragments of Mendicant Bias’s AI were located on the Ark and the Covenant’s holy city of High Charity. When High Charity travels to the Ark in Halo 3, the AI repairs itself and brings itself back together again. It then directly communicates with Chief through the Halo 3 terminals, saying that he was sorry for everything he did and that he would help Chief in any way he could. He even had direct contact with Cortana when she was trapper aboard High Charity.
Skip ahead again to the Halo 5 2013 e3 promo, we see Chief standing before a colossal Forerunner robot rising from beneath the sands. If you read the description of Mendicant Bias’s body when it is buried, you’ll realize that the Forerunner creature in the promo greatly resembles the detailed body of Mendicant Bias when it was buried. Knowing that he couldn’t activate a Halo array, the Master Chief may have released Mendicant Bias on Sangelios in a desperate attempt to stop the Flood. The latest Halo 5 poster shows that same giant robot figure standing upon the ruins of a colony city. It’s possible that Chief unleashed Mendicant Bias on Sangelios so that he could destroy the Flood, but he also destroy Sangelios in the process. Not realizing why, Angent Locke may just know Chief to be the traitor that destroyed Sangelios. The Halo 3 terminals talked a lot about Mendicant Bias, and after everything the terminals foreshadowed for Halo 4 with the Librarian and Didact, it wouldn’t make sense if Mendicant Bias didn’t show up at some point. Not only this, but the Halo 2: Anniversary terminals also talked about Mendicant Bias, further forshadowing his return. All of this fits perfectly with the teasers of Halo 5: Guardians. The fact that Mendicant Bias repaired himself from rampancy, a capability he claimed that only truly strong AIs could possess, also parallels the story of Cortana and how she actually did die from rampancy. Maybe Mendicant Bias also knows a way to restore Cortana, further giving more reasons as to why Master Chief may be dealing with him. I honestly think that there is a very strong possibility that this will be the setup of Halo 5: Guardians. Does anyone think I may be on to something?