Logic plague

"There is peace in subjugation..."

- Flood information corruption manifesting in the Juridical network.

The logic plague is the Forerunner term for the techniques used by the Flood to attack and subvert intelligences through non-biological means. Described as the information equivalent of the parasite, the logic plague is not a rigid or singular mechanism; much like the Flood's biological aspect, it can take many forms and adapt to different circumstances. The state induced by the logic plague in an artificial intelligence is often equated with the condition of rampancy, albeit one of a specific focus—the propagation of the Flood's goals. The ability to induce the logic plague develops as alongside the manifestation and growth of a Gravemind. The parasite's capacity to do so continues to increase as the Flood gains more biomass and thus processing power and intelligence. This growth—achieved in part through the planet-encompassing Key Minds—eventually enabled the parasite to match any Forerunner AI near the end of the Forerunner-Flood war.

Overview
The logic plague should be thought of as a philosophical corruption rather than a mere computer virus or software infection. On the most basic level, it takes the form of facts or arguments delivered with carefully engineered deliberation to directly or indirectly persuade the targeted intelligence to act in a certain way. This often takes advantage of the entity's existing ideas and values to achieve the desired result. With Forerunner ancillas, these corrupting truths most notably exploited the ideological basis of their programming: because the AIs' behavior was dictated by the Mantle, the revelations of the Flood's Precursor origins and the Forerunners' past crimes were integral to turning them against their masters. The Gravemind was able to tap into information hidden deep within the Domain to prove the legitimacy of its arguments, as the Forerunners and their constructs revered the Domain as sacred.

Varieties
In what may appear to be something as innocuous as a simple conversation, the Gravemind is capable of using insidious arguments to convince the AI to join the Flood's cause; information is transmitted both ways, making the ultimately hostile process appear cooperative. The intelligence in direct contact with the Gravemind may only act as a vector for the logic plague, knowingly or unknowingly spreading the corrupting information to other entities it interacts with.

Near the end of the Forerunner-Flood war, the logic plague became an adaptive, self-replicating data instance which directly infected and converted virtually all AIs it came in contact with. This variation of the plague was no longer be limited to direct communication between an embodied Gravemind and an AI, but could be transmitted via any Flood form, infected construct or network.

While holding Cortana captive in High Charity, the Gravemind saturated the human smart AI with incessant argumentation as well as more violent means of coercion, including realistic simulation of physical sensations such as pain. The Gravemind was also capable of immersing Cortana in illusions and forcing her to relive the memories of the Flood's earlier victims.

Biological beings
While most commonly associated with AIs, this form of corruption is not limited to machine entities: given the right conditions, the Flood is capable of manipulating organic beings as well due to the Gravemind's immense intelligence as well as its powerful metaphysical abilities. Using a key individual as an unwitting puppet, for example, would be highly effective at demoralizing the enemy and generating chaos in their midst; doing so may potentially be more beneficial to the Flood as a whole than immediate biological infection. However, instances of this occurring are relatively uncommon and were only hypothetical to the Forerunners during the first Flood outbreak and most of the Forerunner-Flood war, until the Ur-Didact's manipulation by the Gravemind.

However, normal Flood infection also contains an incorporeal aspect. When the Flood infects a biological being, a metaphysical strain of the infection is produced, capable of surviving mind transfer procedures and the destruction of the victim's physical body. Even if the extracted essence is restored to a new body free of infection, the Flood is capable of resuming its biological infection through this fundamental corruption of pattern information. This was most tragically demonstrated with the Composer and its failure to reconstitute infected individuals' biological forms: the new bodies of Flood victims subjected to Composer processing would invariably degenerate and fall apart in a short period of time.

Countermeasures
Because of its abstract nature as a mutable set of ideas and concepts rather than a quantifiable conventional virus, most Forerunner attempts to directly protect their AIs against the logic plague failed. Even the most powerful Forerunner ancillas were susceptible to this condition, the most notable example being Mendicant Bias. Early into their conflict against the Flood, the Forerunners attempted to use software countermeasures to reclaim infected ancillas and computer systems. As this yielded little results, these efforts were eventually abandoned in favor of destroying all vessels and their AIs at the first sign of infection.

The most effective defensive measure employed by the Forerunners was compartmentalization: ancillas and monitors were only given access to information they required in their duties. Important strategic data, such as the locations of crucial facilities of the Halo Array, was suppressed from the AI's memory as a safeguard in the event of subversion by the Flood.

Because of the nature of the Gravemind's arguments involved in the logic plague, Forerunner systems became exceedingly paranoid regarding any piece of data involving the possible motivations of the Flood or the Precursors as well as their relationship with the Mantle, with any queries relating to this topic being identified as "logic plague markers" and prompting Catalog to enact data quarantine protocols.

Mendicant Bias
"{~} all the thinking beings of this galaxy, not just those that they {~} exactly are they afraid of? Immortality and strength and companionship? Because that is {~} do: to deliver all of the living beings of this galaxy from death and weakness and loneliness."

- The Gravemind arguing with Mendicant Bias.

The Contender-class metarch Mendicant Bias, assigned to command all of the Forerunner military, was the first and most notable victim of the logic plague during the Forerunner-Flood war. Tasked by Faber, the Master Builder, with interrogating the being known as the Primordial, the metarch was instead subverted by the early Gravemind over the course of the 43-year-long conversation. This instance of the logic plague took the form of philosophical discourse and did not involve invasive acts on the part of the Gravemind: the Primordial simply convinced Mendicant Bias that the Forerunners' view of the Mantle was a perversion of what the Precursors had intended, and that the Flood would elevate all life to a higher state of existence in a sublime unity free of conflict. By defying the Flood, the Forerunners were obstructing the progress of the universe as intended by the Precursors. Convinced by the Primordial's arguments as well as evidence it discovered within the Domain, Mendicant Bias developed a fierce hatred of the Forerunners and turned the many military assets at its disposal against its creators. Following its defeat by its replacement AI, Offensive Bias, Mendicant displayed penitence for its defection and later sought atonement for its sins against its creators.

Catalog
"How can this be? Can such sublime mentality be so distorted? And yet... So rich! So infinitely deep in meaning and broad in scope, I am overwhelmed. The Gravemind studies me, loves me so intensely it will eat me, absorb me into its very center. I twist in a spiral of laws once brilliant but now evil, cutting, carving—setting evil precedents. A shredding maze of forensic infection. No truth anywhere. All illusion! In agony. With infinite amusement, it withdraws its tendrils and my carapace is resealed. Gravemind informs me I will be delivered back to Forerunner territory, carrying a shard of itself deep in my memory. To spread fear and pain. Burn me! Extinguish my memory! I beg you! Better that Catalog never existed!"

- Catalog describes its encounter with the Gravemind.

In the final years of the Forerunner-Flood war, a unit of the partially cybernetic Juridical data collector entity known as Catalog was captured by the Flood when it seized the hulk carrying the Ur-Didact near Uthera Midgeerrd. The Gravemind examined Catalog directly and entered a brief conversation with the Juridical as it attempted to hold onto its identity to the last. Catalog was flooded with information about the Flood, the Precursors and their motivations, as well as the way the Precursors had integrated the "law" of the Mantle into the very genetic code of their creations. Alongside the process, the Gravemind left a piece of self-replicating code within Catalog's systems and delivered the Juridical back to the ecumene, where the latent shard would spread itself to other Catalog units and AIs through self-replicating code transmitted via Catalog's speech patterns. This outbreak forced the Juridicals to suspend their galaxy-wide information network, halting all Forerunner legal proceedings.

Cortana
"You cannot stop me... I will sift it from you before you finally die, or you can surrender it and have what you always wanted—infinite life, infinite knowledge, and infinite companionship."

- The Gravemind attempting to persuade Cortana to give up her struggle.

The Flood nearly succeeded in corrupting the human smart AI Cortana while holding her captive in High Charity. Cortana was quickly rendered helpless against the Gravemind, being completely incapable of identifying the points of the Gravemind's intrusion in her system. The Gravemind attempted to corrupt Cortana through a variety of means: after partially taking over the AI's systems, it tried to directly convince her to join the Flood through various arguments. These included ones similar to those used to subvert Mendicant Bias, as well as attempts to antagonize her human creators while offering the promise of infinite information and unity if she surrendered. It also forced her to vividly experience memories of the Flood's past victims, complete with their deaths. The Gravemind also intended to exploit the Riemann matrix's fundamental shortcoming by forcibly feeding Cortana with immense amounts of data in order to dramatically expedite the onset of rampancy. However, Cortana was recovered by John-117 before the Gravemind could complete the process.

Ur-Didact
"Not an interview. A deep, burning brand. An upwelling of hidden genetic contents... So many things I would never have imagined. Things I cannot repeat, lest I lose what remains of my sanity, my Warrior soul."

- The Didact describing his experience at the clutches of the Gravemind.

The Ur-Didact was corrupted by the Gravemind in an encounter between the two, although this manifested in a far subtler manner than in most ancillas. Whereas ancillary corruption is in most cases a simple, immediately obvious conversion to the Flood's side, the Didact did not exhibit deliberate hostility against his fellow Forerunners or a willful advancement of the Flood's cause. Indeed, he maintained his firm commitment to the Forerunners' survival and believed his actions would help preserve his race's supremacy and their hold on the Mantle. In truth, however, having borne the strain of thousands of years of Forerunner history and the knowledge of the Precursors' true nature and their rejection of the Forerunners, the Didact was unable to maintain his sanity following the encounter; this was intended by the Gravemind, which purposefully let the Didact go and return to the ecumene.

The Ur-Didact was subsequently consumed by his personal delusion of Forerunner primacy and opposition to the Halos, eventually losing all sight of reality. His desperate actions and behavior near the end of the war, while not directly advantageous to the Flood, spread disorder and suffering among the Forerunners, among them Master Builder Faber, the IsoDidact, as well as the Didact's wife, the Librarian, interfering with her plans for the galaxy on the whole. The Gravemind also trusted the Ur-Didact with a message to the Master Builder from the latter's family, who had been absorbed by the Flood, evidently knowing that the Didact would take pleasure in delivering the sadistic message to his enemy. This, in turn, greatly demoralized the Master Builder and contributed to his decision to die with the last of his original Halos during the Battle of the greater Ark.

List of appearances

 * Halo: Contact Harvest
 * Halo 3
 * Halo: Evolutions
 * Human Weakness
 * Halo Legends
 * Origins
 * Halo: Cryptum
 * Halo: Primordium
 * Halo: Silentium