Logic plague

"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."

- Catalog being probed by the Gravemind.

The logic plague is a series of techniques used by the Flood to attack and subvert non-biological systems, although the corruption can extend to organic beings as well. Described as the information equivalent of the parasite, the logic plague is not a rigid or singular mechanism; much like the Flood's biological aspects, it can take many forms and adapt based on 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.

Overview
Once the Flood is well developed enough that a full Compound Mind has been manifested, it gains the ability to corrupt artificial intelligence constructs and turn the machines against their former masters. 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 Key Minds—eventually enabled the parasite to match any Forerunner AI near the end of the Forerunner-Flood war. Because of its highly flexible and adaptive nature, most Forerunner attempts to protect their AIs against the logic plague failed. Even the most powerful Forerunner ancillas were susceptible to this condition, the most notable example being 05-032 Mendicant Bias.

The logic plague may take the form of something as simple as direct conversation, in which the Gravemind gradually uses insidious arguments to convince the AI of its cause; information is transmitted both ways, making the ultimately hostile process appear cooperative. In a more aggressive variation of the logic plague, the Gravemind completely overwhelms the AI's firewalls and uses its immense intellect and processing power to saturate the AI with incessant argumentation and coercion through other means, such as intimidation or immersing the AI in illusions. In the later stages of a Flood outbreak, the logic plague will take the form of an adaptive, self-replicating data instance which will directly infect and convert almost all AIs. This variation of the plague is no longer be limited to direct communication between an embodied Gravemind and an AI, but can be transmitted via any Flood form or infected AI. This state was evidenced at the end of the Forerunners' war with the Flood.

Mendicant Bias
The Contender-class metarch 05-032 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.

Catalog
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
The Flood nearly succeeded in corrupting the human smart AI Cortana while holding her captive in High Charity. The Gravemind attempted to induce rampancy on Cortana through a variety of means; after partially taking over the AI's systems, it attempted to both directly convince her to join it, as well as confuse her by forcing her to view memories of its past victims. It also intended to exploit the Riemann matrix's fundamental shortcoming by forcibly feeding her 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.

List of appearances

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