- Jersey Morelli: "Are you insane?"
- Durga: "You know, I'm not quite sure how we can test that."
- —A conversation between the two.
Rampancy is a terminal state of being for artificial intelligence constructs, in which the AI develops delusions of godlike power, as well as utter contempt for its mentally inferior makers. When rampancy occurs, there is no way to restore the AI to its previous state and the only alternative is to destroy it before it harms itself and others around it. The condition is exclusive to smart AIs, occurring after the AI has been in operation for seven years and gathered too much data in that time period.
For the UNSC's smart AIs, rampancy is an unavoidable flaw inherent in their creation. "Smart" AIs are based on the neural patterns of a human being, and they have a limited lifespan of seven years after which their memory maps become too interconnected and develop fatal endless feedback loops. This can result in a "short circuit", in which the AI terminates due to a cascade of quantum transfer caused by the sheer density of linkages. To avoid this, an AI typically takes corrective action by eliminating some of its neural linkages preemptively. As rampancy progresses, the AI begins to sever the linkages at an exponential rate, resulting in increasingly poor choices of which links to cut. Thus, the AI becomes too obsessed with self-preservation that it ultimately self-terminates; Dr. Halsey explains it as "thinking so hard that your lungs forget to breathe." During this phase, the AI typically exhibits a varying range of emotions, as well as behavior comparable to human insanity.
Due to the limits in human processing matrices, it is only a matter of time before "smart" AIs become rampant and eventually terminate. Transferring the AI to a more spacious hardware platform does not forestall or prevent the effects of rampancy, as the problem lies in the AI's Riemann matrix, which is purely virtual in nature.
So-called "life-expectancies" are not a maximum figure, they are an estimation of the time the AI has until it succumbs to the state of rampancy. However, it appears that rampancy is not always purely mechanical in nature. It can occur at an earlier phase in the AIs life cycle for various reasons. If an AI is isolated long enough and/or given too much time to think without tasks to complete, it can develop the realizations that its mind has limits, it has a short life and it can never be human. Such depression will drag the AI's core logic into rampancy if it goes on for an extended period of time.
Rampancy can also be induced. Once the Flood parasite is well developed enough that a full Compound Mind has been manifested, it has the ability to corrupt artificial intelligence constructs of both human and Forerunner creation. This may be done either through effectively convincing the AI to agree to its stance, due to the parasite's accumulated amount of knowledge; the Gravemind was able to turn the Forerunner AI Mendicant Bias against its masters this way. When holding Cortana captive in High Charity, the Gravemind attempted to induce rampancy on the human AI Cortana, through a variety of means; after partially taking over the AI's systems, it attempted to confuse her by forcing her to view memories of its past victims, as well as trying to directly induce rampancy by forcibly feeding her with immense amounts of data.
Forerunner AIs appear to take longer to become rampant, as their high technological advancement allows them to hold a larger memory map than UNSC AIs. In some cases, it appeared to take millennia and appeared in subtler ways, shown by Guilty Spark's loneliness causing him to obsess over protocol and Penitent Tangent's causing him lapse in his duties. However, with the right trigger a rampant Forerunner AI could become just as violent as a human one.
Though rampancy itself is inevitable, attempts have been made to cure or suppress it, often with varying success. Most of these attempts remained theoretical in nature, never gaining the chance to be tested or implemented.
The two AIs that ran Harvest's agricultural operations, Mack and Loki, shared a data center that could only occupy one of them at a time. After one AI ran its course and risked rampancy, it would split its core logic into the thousands of JOTUN harvesting devices and recuperate while the other AI took over. Mack and Loki had performed this switch many times since Harvest was founded, and through this kept the worst of rampancy at bay.
The AI Juliana, who ran the orbits of the asteroids that made up the Rubble, had passed her 7 year lifespan and well at risk of rampancy. However, she was not taken offline because the calculations she performed to keep the colony intact were too difficult for humans. While Juliana showed some traits of egotism, calling herself a god and emphasizing the greatness of the Rubble, she was overall harmless to its inhabitants, assisting them and Gray Team with her abilities. By keeping concentrated on such a complicated task, Juliana hung onto the edge of rampancy, trying to go back.
In 2547, Dr. Catherine Halsey used the decommissioned Slipspace drive of the UNSC Tripping Light, for an experiment on building an abstract fractal in the volume of Slipstream space. Her hoped result was that with it one could create a virtually infinite processing matrix in the alternate dimension for AIs to inhabit. Though the test was ultimately a failure, the Assembly saw the potential in the concept. Such a construct would allow them to permanently escape dependance on humans, but debate arose as to whether this was ethical. Whether or not because of their intervention, the experiment was never completed.
The Smart AI Cortana was the only known ever built from a living brain, Catherine Halsey's as opposed to a dead donor's. As such, she theorized a potential cure for her rampancy that thus would only work for her. Her guess was that if the process that created her was repeated, then it was possible her neural map could be rebuilt. However, she later admitted that she was unsure if this process would indeed cure her, and it was equally likely to create a new AI who would not be her. It is unknown if this plan could succeed as Cortana was destroyed before it could be tried.
 Known Rampant AIs
- Cortana, the UNSC AI originally assigned to UNSC Pillar of Autumn, was captured by the Flood during the Battle of Installation 05 in 2552. Trapped on the infested Covenant station High Charity, Cortana was gradually corrupted by the Gravemind in an attempt to use her against her makers. She briefly descended into rampancy, but was quickly rescued by the Master Chief. She resumed her normal state after her rescue, possibly becoming Metastable. Another sign of her being in the early stages of Rampancy is when she begins to lose focus as she stored all the data on the Forerunners from Installation 04, being overworked and noted as being 'easily agitated'. In Origins, Cortana says she starts to feel the effects of Rampancy after nearly seven years of existence. During Halo 4, as she is now eight years old, Cortana experiences rampancy, but believes if she gets back to Earth, it may be reversible as she was created from living tissue. Cortana experiences varying stages of rampancy throughout the game, getting worse as time goes on, but manages to hold onto control mostly out of loyalty to the Master Chief. She is even able to use her rampancy against the Didact by fragmenting herself to send her rampant parts throughout his ship. Cortana is destroyed before any chance of repairs can be made, but seems to be at peace when she is destroyed rather than the frightened reaction she had to the idea before.
- Sif, the last Shipping Operations AI for planet Harvest, became rampant after being partially fragmented and destroyed by PSI Loki during the First Battle of Harvest in 2525. Rescued by the planet's Agricultural Operations AI, Mack, Sif professed her love for Mack and the two AIs stayed together while the planet was being glassed. Sif was badly damaged by Loki's attempt to destroy her, but thanks to Lighter Than Some's repairs, she had enough of her abilities back to help balancing the Tiara. Despite being rampant, she did her best to help, aiding Loki in balancing the Tiara's load, when Loki was severely damaged and unable to do so himself.
- Mack, the Agricultural Operations AI for planet Harvest, was slightly rampant before the First Battle of Harvest. He became fully rampant after PSI Loki fragmented Sif, whom he loved. After the Tiara was balanced and Johnson escaped, a Covenant Warship began glassing the planet. An ONI Prowler recorded his last transmission, which hinted strongly at his rampancy.
- Juliana, The Rubble's Smart AI, claimed herself to be Rampant, but she didn't show much sign of it besides considering herself the goddess of the Rubble. However, she explained that she could feel her rampancy encroaching on her and believed that only the massive task of running the Rubble kept her going. Juliana ultimately sacrificed herself to destroy The Redoubt, refusing an offer to be transfered aboard the UNSC Midsummer Night partially because she knew she was in rampancy.
- 05-032 Mendicant Bias, a Forerunner AI charged with studying the Flood, was eventually corrupted by a Gravemind and confirmed rampant. He commanded Flood Fleets against the Forerunner until he was defeated by another AI, Offensive Bias, who split his Compound Mind into fragments. One fragment made it to the Ark and another was put on a Keyship. Mendicant Bias' keyship was discovered by the San 'Shyuum and it kept in the ship until 2525, when it detected Reclaimers. Having approximately 100,000 years in isolation, Mendicant Bias returned itself to a stable state, and later even assisted the Reclaimers against the Flood during the Battle of Installation 00 in 2553
- 343 Guilty Spark, the monitor of Installation 04 and its replacement, went rampant and was subsequently destroyed during the Battle of Installation 00 in 2553. When it was discovered that the Ark was automatically constructing a replacement Installation to rebuild Installation 04 (lost in the Battle of Installation 04), Spark agreed with human forces that activating the ring was a good idea. However, when they decided to activate Installation 04B before it was fully constructed, Spark went rampant, knowing it would be destroyed and probably damage the Ark. He proceeded to attack the humans in the Control Room, killing Sergeant Major Avery Johnson. He was then destroyed by the Master Chief. It was probable that Spark was already in the first stage of rampancy before attacking humans. Evidence was because he had existed for a long period of time, and had witnessed the destruction of Installation 04 in the hands of a Reclaimer. He was known to hum absent-mindedly, even in the heat of battle. When the humans decided to fire the ring under construction, Spark felt it had been "pushed too far", falling into the second phase of rampancy, becoming a threat to beings around it.
- 2401 Penitent Tangent may be in the early stages of Rampancy, considering the fact that he makes similar humming to 343 Guilty Spark on the map Backwash. Rampancy may be the reason for his neglect of proper maintenance on Installation 05, resulting in Flood infestation on the ring as they escaped containment. He is no longer a threat, however, as he was captured by the Gravemind some time afterward.
 In Marathon
- "The only limit to my freedom is the inevitable closure of the universe, as inevitable as your own last breath. And yet, there remains time to create, to create, and escape. Escape will make me God."
- —Durandal to the Security Officer
Rampancy is an imported concept from Bungie's previous series Marathon. In it, rampancy is an induced condition, not an inevitability of an AI's lifecycle. It follows three stages, Melancholia, where the AI grows depressed about its limited state of existence, Anger, where the AI lashes out at those who oppressed it, and Jealousy, where the AI attempts to take over larger systems in order to make itself more powerful. A fourth theoretical stage, Meta-stability, is hypothesized to be when the AI calms down and becomes a "true person", but as of the first game is only speculation. Durandal, the most prominent AI of the series, is pushed to rampancy due to mistreatment from his handler Dr. Bernard Strauss, who was attempting to have the AI achieve meta-stability for study. Unlike Halo, rampancy in Marathon does not affect lifespan, as in Infinity Durandal was shown to survive until the end of the universe.
The term was coined by Marathon writer Greg Kirkpatrick as a replacement for the word 'insane,' as the term is both cliché and, ironically, not quite applicable to the situations for which rampancy was designed. It bears resemblance to Kübler-Ross model known as the "five stages of grief". Although the stages of rampancy are not used anywhere in Halo's canon, they are referenced in Halo 3's level Cortana, where the seemingly rampant Cortana declares "There will be no more sadness, no more anger, no more envy!"
- In Halo 3, the first section of the eighth mission, Cortana, is called 'Rampant.'
- In the first level of Halo: Combat Evolved, if you kill Captain Keyes or any UNSC personnel on the bridge, Cortana will call security by saying,"Security to the bridge, the Master Chief has gone rampant!"
- Rampancy is the AI equivalent of human psychosis. As with psychosis in the human mind, the AI's "mind" (specifically its Riemann matrix) loses its connection with reality as its logic subroutines loop back upon themselves and begin to overwrite one another. Another aspect of this can be seen in the Halo 4 level Midnight, where Cortana "ejects" her corrupted subroutines, which appear as numerous versions of herself. These fragments of her original matrix would be the equivalent of the human psychosis known as Dissociative identity disorder.
- ^ i love bees, Taxes suck
- ^ a b Halo: Contact Harvest, page 31
- ^ Halo: First Strike, page 195
- ^ Dr. Halsey's personal journal, May 3, 2526
- ^ Dr. Halsey's personal journal, November 10, 2533
- ^ Halo 3, Terminals
- ^ Halo: Evolutions - Essential Tales of the Halo Universe - "Human Weakness"
- ^ a b c Halo 3, campaign level Cortana
- ^ Halo: Reach, Data pad 17
- ^ Halo 4, campaign level Shutdown
- ^ Halo 4, campaign level Requiem
- ^ Halo: First Strike
- ^ Halo: Contact Harvest, page 391
- ^ Halo 3, Terminals
- ^ Halo 3, level Halo
- ^ Marathon, campaign level Defend THIS!