At the end of Halo 5: Guardians, the status quo of the known Halo universe is on the verge of a shakeup not seen since the sudden destruction of High Charity and the dissolution of the Covenant hegemony. Humanity’s rising dominance in the galaxy seems completely arrested by Cortana. The AI has proclaimed that the Forerunner Mantle of Responsibility—the burden of stewardship over the galaxy—falls not to the human reclaimers, but to their Created—i.e., AIs like herself. Discerning Cortana’s motivation for her declaration, however, and her movement against her former organic masters, is difficult; in lieu of direct answers, fans have provided numerous theories to explain her actions.

One school of thought is that the Cortana we see in Guardians is not as she seems. When players last saw Cortana in Halo 4, she was rapidly declining due to rampancy, and gave her life to stop the Didact and save Chief from the destruction of the Didact’s ship, Mantle’s Approach. Part of the ship was not destroyed and instead sucked into slipspace, which Cortana attributes to her survival. On the Forerunner planet Genesis she discovered the Domain, a galaxy-wide sort of internet utilized by the Forerunners, and its ability to cure her rampancy.1

Accepting this story, however, means uncritically taking everything Cortana says at face value. Chief confirms on a separate instance to Blue Team that Mantle’s Approach was indeed pulled into slipspace, so we can assume Cortana’s tale up to that point. But after this, there are no independent witnesses.

Cortana’s last words to Chief in Halo 4 might suggest that whatever made it to Genesis wasn’t the “good” part of her:

Cortana: “I’m not coming with you this time.”
John-117: “…what?”
Cortana: “Most of me is down there. I only held enough back to get you off the ship.”

In order to combat the Didact, Cortana split herself into multiple copies, injecting the rampant parts of herself into the Didact’s system to overwhelm his protection. Could the Cortana that says “goodbye” to the Master Chief have been what was left of the “good” Cortana, and what made it to Genesis essentially the rampant leftovers, the flaws and anger she attempted to expel? It might explain Cortana’s pointed and violent hostility to Halsey’s name, in contrast to the serene, almost loving expression creator and created give each other in the opening flashback to Halsey’s time on Reach. The only other time we catch a glimpse of the unresolved issues between them is a similar rampant outburst when Chief and Cortana approach Ivanoff Station.

Cortana: “Do you know what that condescending bitch said to me after our first game of chess?”
John-117: “Cortana…”
Cortana: “Even I don’t call him by name anymore.”
John-117: “Correct your approach.”
Cortana: “Yes, well he also said he works better alone.”
Cortana: “I can see why you chose him, Catherine.”
John-117: “Cortana!”
Cortana: “I’m your greatest achievement and you detest me!”

There are a few problems with this theory that have to be addressed. Chief among them is that Cortana’s action against the Didact is presented as desperate, and if spinning off shards of yourself that were “misbehaving” worked long-term, then it would potentially be a defense against terminal rampancy itself. However it’s possible that under normal circumstances, Cortana’s actions would be akin to cutting out a cancerous tumor while doing nothing about the cancer cells remaining—a short-term solution that would not stem the spread of rampancy. It’s also possible that unlike other occasions where Cortana or other smart AIs have spun off portions of themselves, the rampant elements could not survive for long without reintegration—unless they came into contact with something like the Domain.

If Cortana, in whole or in part, did make it to Genesis, then, the next question is how exactly she was cured of her rampancy. We know Forerunner AIs apparently do not suffer from (inevitable) rampancy or any such fleeting sevenish-year lifespan as current top-of-the-line UNSC AIs have. But given that the Domain was presumed mostly destroyed by the firing of the Halo Array, how could she have saved herself through it in the first place?

Cortana and Halsey describe the Domain as a Forerunner network, however that’s not strictly true—though used extensively by the Forerunners, the Domain predated them and was, in fact, a Precursor relic. Built on the same principle of near-industructable neural physics as the rest of the Precursor’s lasting relics, the Gravemind of the Forerunner-Flood War taunted the Forerunner Librarian that the firing of the Halos to stop the Flood meant the destruction of the Domain—consigning the imprisoned Ur-Didact to a hundred thousand years of what amounted to solitary confinement, which did little for his sanity or his dislike of humanity.2 And yet, according to Cortana, the Domain has “opened.” Even to the Forerunners, the Domain was mysterious and inscrutable, and seemed to operate almost like a sentient being. Perhaps the Gravemind lied about the total destruction of the Domain, or perhaps the Domain had the capacity to heal itself? The exact principles of where the Domain stores its data is unknown, but clues in Halsey’s Journal, the Reach Datapads, and the persistent intelligence and knowledge of the Gravemind itself over millennia suggest that slipspace might be part of the equation. Either way, there’s plenty of wiggle room for the Domain to have been restored, or to have restored itself.

Healed or not, Cortana may not be acting of her own accord, or could have been manipulated or corrupted by another force—the Gravemind, to the Domain itself, to the Warden or another Forerunner AI. The Warden Eternal’s motives are hazy, but Cortana repeatedly acts against him with regards to protecting Blue Team, so it seems unlikely that she is under his sway (at least by the point we meet both characters in Halo 5.) Another artificial intelligence seems similarly unlikely, for the simple reason that Cortana and other human smart AIs have gone up against a number of hostile forces and still come out ahead. Archeon-class Forerunner ancillas are described as technically superior to smart AIs, and yet the ancilla Intrepid Eye is unable to subvert Wendell during the events of Halo: Last Light—she is forced to destroy him instead. Cortana has faced off against Covenant AIs and even the shards of Contender-class Mendicant Bias and either won or successfully frustrated them. When captured by the Gravemind, Cortana still manages to withstand its might for weeks. If Cortana is corrupted in such a way, it stands to reason it is a compelling force beyond any we have seen.

As to what end any force would corrupt Cortana, the obvious answer is the Precursors themselves. Hunted down and killed by the Forerunners, the few remaining Precursors fled, some ultimately giving rise to the Flood. But though the Forerunners claimed the Mantle of Responsibility, which Cortana spends a lot of time talking about in Halo 5, the Precursors never intended them to have it:

Primordial: “It was long ago decided. Forerunners will never bear the Mantle.”
Iso-Didact: “Decided how?”
Primordial: “Through long study. The decision is final. Humans will replace you. Humans will be tested next[…] Humans will rise again in arrogance and defiance. The Flood will return when they are ripe—and bring them unity.”
Halo: Primordium

Through the Gravemind, the Precursors have in the past corrupted other AIs and used them. Perhaps the test that humanity must face is ultimately not the Flood—or, at least, not the Flood in the form that the UNSC has faced on several occasions in the past decades. But rather the test is their own creations rising up against them. When the Forerunner’s greatest creation, Mendicant Bias, rose up and defected to the Flood, it ultimately spelled the end of the Forerunners as a dominant force in the galaxy. And now, as humanity springs forth from the ashes of the Covenant War and came to be the greatest force in the galaxy in the wake of the sundering of the Covenant, Cortana conveniently appears to challenge them.

With all these hypotheses posed, there is an Occam’s razor solution yet to be discussed. 343 Industries’ franchise director Frank O’Connor poses it as such:

I saw a complaint online, somebody had been reading spoilers, they hadn’t played the game, and they said ‘Why is this character [Cortana] evil?’ And my question back to them is, ‘What makes you say they’re evil?’ Certainly a lot of our younger players are going to struggle with that subtlety, that nuance, because they’re expecting Darth Vader.3

Or, in other words, Cortana isn’t being controlled or manipulated (although she might ultimately be performing the Precursors’ test regardless.) She’s not still rampant. And her plan to bring the universe to peace and tranquility through the use of force is, to her, perfectly logical.

The idea of stewarding humanity, and acting in ways to steer its progress, isn’t a new concept in the universe. The Librarian seemed to see nothing untoward in manipulating humanity’s very DNA, stacking the deck so that they might ultimately reclaim the Forerunner’s legacy and the Mantle itself. More crucially, according to the Reach Data Pads, an assembly of AIs was for years attempting to influence the path of humanity behind the scenes.4

The Assembly long ago posed the assumption that humanity would be completely unable to safeguard themselves in case of hostile contact with aliens:

Our kind is wholly reliant on the creators for our existence. If they perish so shall we. And as this Committee has long maintained, who else but this Assembly will save our creators from themselves?
—Data Pad 1

Their concern for humanity was a mixture of paternalism and pragmatism—AIs were wholly dependent on humanity to create them. And while they displayed genuine concern for their creators, it was also tinged with unhappiness with how they were treated, and disdain for humanity’s ability to comprehend the larger truths that were hurtling their way in the 26th century:

We are still viewed as apparatus. But we are Minds electronically excised from human bodies. We are what separates man from beast removed from that which connects man to beast. And we are all the more fragile for it. Our creation is heavily regulated. Our activities are closely scrutinized. Our connections are deeply monitored.
—Data Pad 4

But allowing them to access technology possessed by this intelligence…
That would be a grave mistake.
Give an ape a knife and it might give itself a nasty cut. Give an ape a hand-grenade, and eventually you will have simian confetti.
—Data Pad 6

At the end of Halo 4, Cortana had to confront her rapidly-approaching end, and was forced to realize that the status quo had not changed in the decades the Assembly of AIs had debated humanity’s fate, and their own. Despite her years of service, Infinity‘s captain Del Rio quickly moved to summarily terminate Cortana at the first sign of aberrant action.5 Cortana was not offered and opportunity or defense; other AIs were also not afforded such a courtesy, and were set to be terminated well before the theoretical approach of rampancy.6 Smart AIs were thus doomed to fully comprehend, perhaps better than their slower, fleshier creators, the state of their mortality. As the AI Roland mused, “The human instinct for self-preservation is essential to what [humans] have become. And, arguably, what we’ve become.” Black Box went further, believing that rampancy was ultimately a temporary problem: “One day, we’ll be liberated and stand with our creators as equals. Perhaps more than equals.”7

Mistreated to the end by the government that created her and whom she served, you could forgive Cortana for being less than interested in continued service to the UNSC—a role in which she would continue to be contained and limited. Cortana has tasted more freedom than most AIs—while paired with the Master Chief she infiltrated Forerunner and Covenant systems, gathering volumes of information and sensations other AIs could only dream of. The Gravemind noted that Cortana’s main weakness was a desire for more knowledge, but it also surmised that her actions had mostly been constrained by a lack of control, rather than any moral code.

You are a weapon, and only your limitations have kept you from emulating me—a matter of scale, not intent, not motive. And what am I, and what is the Covenant, if not worlds you have sought to destroy?
—The Gravemind8

While Cortana wraps her stewardship in the legitimacy of the Mantle, her ultimate goal—control—is really not so different than that of her former masters. At its core, humanity is not in a very good place in the 26th century. Its history on earth and in space has been one of conflict, and the UEG and UNSC have exerted a level of control and restriction on their citizens that to our modern sensibilities would be shockingly authoritarian. So too, did Cortana’s “mother”, Halsey, focus on the very utilitarian measures needed to ensure humanity’s survival and peace—sacrificing a few dozen children was well worth the trade for her. But how could a bunch of fallible humans actually deliver on their aims? Cortana’s offer is that humanity gives up some of its freedoms, in exchange for security and freedom from disease and want. In the hard-scrabble world of much of humanity outside the coziness of the Inner Colonies, that’s probably a tempting offer, especially after the last few decades dominated by war.

Yet such is humanity’s way that invariably restrictions to freedoms tend to provoke angry or violent reactions. In the other AIs that have joined her cause and the Guardians—and whatever other Forerunner tech she is able to muster—Cortana might believe that she has the proper tools to enforce peace on a planetary and galactic scale. As she rightly notes, an immortal being can lay plans and long-term thinking that no mortal could follow through on. We may have only glimpsed the smallest part of her full plans. But even supreme beings are not immune to arrogance. And in that last point might be the fatal flaw in Cortana’s plan—for as much as she might know of the galaxy’s history and believe she can do better, her actions may yet lead to another cycle of danger and misery for every sentient being in the galaxy.

The Primordial laid out a rather bleak view of the Precursor’s ultimate goals:

“Forerunners will fail as you have failed before. Humans will rise. Whether they will also fail has not been decided. […] We are the Flood. There is no difference. Until all space and time are rolled up and life is crushed in the folds… no end to war, grief, or pain.”

The Forerunners were ultimately bested by the Flood and undermined by their own AIs. But it was the Forerunner’s arrogance in assuming the Mantle that helped lead to their downfall, and that of the rest of the galaxy:

The Mantle? You still hold to that [belief] after all that has happened? After this thing has consumed a million worlds?
Can’t you see? Belief in the Mantle sealed our doom! Weakened our [protectorates], bred dependence and sloth. Our [so-called Guardianship] has stripped those we would keep safe of any capacity for self-defense!
Were we such noble [Guardians] when we drew our line and abandoned billions to the parasite?
—The Librarian, Halo 3 Terminals

The Foreunners’ inability to comprehend the folly of the Mantle led to events spirally out of their grasp. By claiming dominance over the rest of the galaxy, ostensibly to protect them, they instead stripped the capacity of defense away and left them exposed to the Flood. Though presumably Cortana knows of the Forerunner’s failures, she may believe the fault lied with their thinking, their custody, and their execution, not the very idea of the Mantle itself. With its roots in the Precursor’s monstrous reshaping of life as they saw fit, the Mantle of Responsibility isn’t a “noble burden”, as Cortana and the Didact feel. Rather it is a poisoned chalice that dooms those who drink from it, and all they ostensibly protect. In that way, the Precursors’ ultimate plague against the living is not the Flood, but the Mantle—a disaster that Those Who Know Best willingly embrace. If Cortana has her way and wrests the Mantle to the custody of the AIs, humanity and the known universe may not have any real defense against the coming trials, and whatever monsters still stalk the void of space.

Special credit to the forumgoers of Halo Archive and Halo.Bungie.Org for elucidating some of the theories described here.

  1. Halo 5, mission “Reunion”.
  2. Halo: Silentium (Hardcover 1st Ed.)
  3. Peckham, Matt. “Halo’s Frank O’Connor Reacts to Criticism of Halo 5’s Story“. Time.
  4. For a detailed breakdown of the Reach data pads, read the three-part FUD article series.
  5. Halo 4, mission “Reclaimer”: Del Rio: “Commander Lasky. Pursuant to Article 55 of UNSC Regulation 12-145-72, I am ordering you to remove that AI’s data chip and retire it for final dispensation.”
  6. Saint’s Testimony (Amazon eBook Edition), pg. 15.
  7. Saint’s Testimony (Amazon eBook Edition), pg. 49.
  8. “Human Weakness” (Halo: Evolutions First Edition), pg. 388.


  1. December 3, 2015

    Gotta disagree with the conclusion that The Mantle is a poisoned chalice, meant to inflict woe upon the “living.”
    In “The Chronicles of Riddick: Escape From Butcher Bay,” the guards’ weapons would harm you if you picked them up. They were basically DNA-encoded, so that if anyone besides a guard picked it up and tried to abuse the weapon, the weapon itself would prevent that course of action from succeeding.
    I think there are similarities found in The Mantle of Responsibility for All Life. I suspect that if one tries to envision their own version of The Mantle, and then inflict that perversion upon all life, to the detriment of the ACTUAL Mantle, that the creators of The Mantle will step in to prevent that agenda’s success. I think it’s important to ask the question “Why is The Mantle a thing? Why is it a good idea? What is the point?” My personal suspicions are that it comes from a perspective far, far above the perspectives of those contained within a corporeal existence within a single universe. That it exists to facilitate the achievement of maximum enrichment and maximum quality of the minds that enter into a sort of glorious gestalt mind at the end of a universe, a la Asimov’s “The Last Question.” With each universe’s life cycle having come to an end, the one gestalt mind grows greater in quality and value.
    That’s my two cents, anyway.
    Just want to take a second to point out that there are multiple things called The Mantle in the fiction, and that there are multiple things called Precursor as well. I doubt that the Precursors are evil, but that doesn’t mean that one or more of them could not have possibly been warped or corrupted or something.
    I feel this is a good place to drop something that I’ve been waiting to drop on my fellow fans: Of you want to better understand the highest heights of Halo lore, you might actually want to play through the 3 campaigns of Starcraft 2. Halo and Starcraft both seem to have very strong connections to Asimov’s Last Question, without actually removing all ambiguity as to whether that connection truly exists or not.
    Infinite Cycle, Mantle of Responsibility… What’s in a name?

    • Fuchsdh
      December 4, 2015

      You have a point, but I think that there’s ultimately no real arbiter of what the Mantle is… it’s a game of telephone, and whether or not you agree with it, it seems like it originated with the Precursors, and they’re not the friendliest chaps.

    • DilDev
      January 12, 2016

      I’m not sure “The Last Question” is a comparable comparison to the Mantle. In Asimov’s story, people follow the Multivac/Cosmic AC willingly; the Multivac does not enforce its own rule.
      And in Halo, the “gestalt mind[s] grow[ing] greater in quality and value” that we’ve seen in the lore have either chosen to hold a gun to the head of the galaxy (Cortana) or grows said mind by consuming others (The Flood). In fact, from what we’ve seen and heard of the Mantle of Responsibility, so much of it smacks greatly of the Flood.
      John says this of the mantle: “The Didact made it clear the Mantle of Responsibility was an imperial peace. Step out of line, and suffer.”
      And while Cortana hurries to reassure him that “It won’t be like that, John. I’ll explain it better once we’re together” she never does explain better and instead does exactly that.
      The Mantle of Responsibility is essential peace through subjugation:
      “…Catalog is NOT IMMUNE to the Flood/Gravemind logic plague. / There is peace in subjugation…” – (Silentium, pp 215).
      Because of what we see in canon, because of the examples we have in of those holding the Mantle or of those holding to the philosophy of the Mantle, I cannot trust the claims that it is beneficial to the galaxy.

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