From Halopedia, the Halo wiki
The Domain was an immaterial reserve of knowledge and collective experience viewed by the Forerunners as the essence and living expression of their culture and history. In addition to the ancestral impressions and memories contained within, information could also be stored in and retrieved from the Domain for practical purposes. There was a mysterious quality to the Domain; despite its ubiquity in Forerunner culture, its exact nature or origin remained largely nebulous even to Forerunners. Due to these abstruse aspects, the Forerunners treated the Domain as something mystical and transcendent; it was regarded with reverence and connected to the Forerunners' religious beliefs.
Forerunners used the Domain via various technological systems, most commonly their personal armor. While the artificial world of Genesis served as a primary link into the Domain, connecting to the Domain was possible for mature Forerunners anywhere within the Forerunners' sphere of influence, regardless of distance. This was typically accomplished via an intermediary, most commonly a personal ancilla as well as Haruspis. A Cryptum also acted as a mediator between a Forerunner and the Domain; meditative xankara in a Cryptum provided a much clearer and more unrestricted experience of the Domain than contact when awake. Dedicated terminals were also sometimes used by ancillas to connect to the Domain when on Forerunner worlds or installations.
The Domain was central in major Council proceedings, such as a political trial. Among other things, it was used by judges to store archival and accounting material, assess precedent, and to verify witnesses' testimonies. Ancillas routinely accessed data within the Domain, and Contender-class AIs such as Mendicant Bias were connected to the Domain upon their activation.
Haruspis and its associates were Forerunners dedicated to studying the Domain. Haruspices also sought out specific information within the Domain on a Forerunner's request. The construct known as the Warden Eternal also served as a protector for the Domain.
The Domain had its own set of rules and restrictions it rarely violated. Forerunners could only view information that they already knew or what was otherwise available to them, namely data stored in the Domain in recent times. The deepest records contained in the Domain could not be accessed, due in no small part to the fact that information in the Domain was not static: the records changed over time as new visits by newer generations laid new layers of information which settled into its own patterns and sought to make itself more complete. These changes were regarded as sacred, and were never reversed or corrected. Over long periods of time, older information in the Domain ultimately became unintelligible to all Forerunners, even Haruspis. Due to these anomalies, the Domain was not commonly used as means of communication; messages could be inexplicably altered on the way.
The Domain retained partial impressions of individuals who had visited it in earlier times. Forerunners connected to the Domain could access these essences, although they only manifested themselves as abstract echoes like emotions or memories as opposed to complete personalities. The Forerunners also viewed the Domain as a form of afterlife, believing that the "essences" of individuals passed into the Domain upon death.
An experience in the Domain could be visualized as a journey through an infinite series of hallways, corridors and caverns, with abstractions of records and memories lining and illuminating their walls. Bornstellar Makes Eternal Lasting described his initial contact with the Domain as initially "deep" and "appropriately shapeless", but it soon took form, shaping into a coherent whole, like a building of majestic yet indefinite architecture, around his consciousness. The Domain appeared to be somehow related to a Forerunner's subconscious; Forerunner scholars theorized that the dreams of early Forerunners accessed the "ground" that the Domain was built upon. The Didact mentioned that the Domain sometimes had a "broken-mirror aspect", making it appear as if one's experience of visiting the Domain was the product of their own mind.
As of the final years of the Forerunner-Flood war, no human was known to have accessed the Domain or left their impressions within. Because of this, the Didact considered the Domain incomplete, as it lacked the knowledge that could be used to predict the humans' reactions in battle.
The Domain itself was an aware, conscious entity with a will of its own. While this aspect of the Domain appears to have been a common belief among Forerunners and ingrained in their idioms and phrases, the Juridicals did not formally recognize the Domain as any form of being or awareness. The Ur-Didact noted that while the Domain kept certain topics secret by nature, it had an urge to spread knowledge and inform Forerunners when they acted against its will. It was not, however, capable of communicating its thoughts directly. Instead it expressed itself via the emotions of the memories and impressions it contained. For example, before the end of the Forerunner-Flood war, the Domain was ingrained with a looming sense of sadness of the impending end of Forerunner history and with it, the Domain itself.
After Boundless' research was suppressed, around 1,100,000 BCE, Haruspis took note of the Domain seemingly favoring her by attempting to push her studies to the fore and make the information known. When accessed by a Juridical looking for information on the Precursors, the Domain itself requested to testify to the Juridical, who disbelievingly asserted that the Domain was in no way a sentient being. Haruspis rebuked the Juridical for his ignorance on the nature of the Domain, just before the Domain itself overwhelmed the Juridical with knowledge.
In certain cases, the Domain has established contact with Forerunner individuals on its own accord, without technological aid like an armor's interface or a Cryptum. When Bornstellar approached Janjur Qom, the Domain opened to him for a brief instant. Through this experience, Bornstellar felt the essences of everyone who had ever visited the Domain reach out to him and an unknown voice urging him to preserve the history of the Forerunners, which would soon come to an end. Years later, in the final days of the Flood war, the Librarian felt that her decision to use herself as bait to draw the Flood to Erde-Tyrene was born of the Domain's influence directing her. This, in turn, bought the IsoDidact enough time to prepare and fire the Halo Array.
The Domain was created by the Precursors over 500 million years before the end of the Forerunner civilization, as a library of experience and cultural exchange totaling to a hundred billion years' worth of knowledge. It was stored within the Milky Way galaxy, sheltered by the Precursors' neural physics-based architecture. The accessible aspect of the Domain was an immense field generated by this reserve. The Forerunners first accessed the Domain with the aid of Abaddon, a construct the Precursors had ostensibly left behind to guide the Forerunners. Over time, the Forerunner capital world Maethrillian was built around Abaddon's core and the Precursor intelligence was sealed in the Mysterium, becoming a closely-guarded secret known only to a handful of Forerunners. Tales of Abaddon eventually gave rise to the legend of the "Organon", a fabled Precursor relic connected to the Domain capable of controlling all Precursor artifacts.
During the final years of the Forerunner-Flood war, the Domain suffered frequent outages, often becoming inaccessible for all Forerunners. This was a result of the Halo Array being moved across the galaxy, and the resulting buildup of space-time debt due to the enormous amounts of particle reconciliation required to correct the breaches in causality. These difficulties would intensify in the coming years, as the Flood began to access neural physics and with it, the Domain itself. The Domain also became more reluctant to accept information for storage, and the information within became more confused. By the last year of the conflict, the Domain had become completely inaccessible to Forerunners and ancillas, with Haruspis completely absent from the network.
At the close of the Flood war, the Gravemind conveyed a message to the Librarian via the essence of Forthencho, revealing the true nature of the Domain as a Precursor construct. As a result, the Librarian came to the conclusion that the Domain was, essentially, the mythical Organon. By activating the Halo Array—which would destroy all Precursor creations by dismantling neural physics—it would be ensured that the Forerunners' history would be lost. The Librarian was shocked to learn this, and sadly realized that without the Domain, the Ur-Didact, locked in a Cryptum on Requiem for his genocidal actions against humanity, would be condemned to spend the ages ahead of him in complete silence, dwelling on his own insanity and rage.
Over a century after the Array's activation, the Bornstellar Makes Eternal Lasting, formerly the IsoDidact, viewed a recording from the Librarian relaying the Gravemind's message. He shared the recording with Chant-to-Green and both realized the consequences of the loss of the Domain. Understanding what that the Domain needed to be somehow repaired, Bornstellar, Chant, and a group of other Forerunners journeyed to Maethrillian to accomplish this. Along the way some of the group encountered Abaddon, now active within the Capital and intent on judging the Forerunners for their crimes. Abaddon killed four of the Forerunners in the group, subdued another, and took over the remaining three Forerunners' ancillas, attempting to lock down their armor. As a last-ditch effort, the Lifeworker Growth-Through-Trial-of-Change inserted the artifact known as the deadbolt key into Abaddon's physical core. While Trial was instantly vaporized in the process, her act seemingly restored the Domain, apparently using the Lifeworker's biological pattern as an initial "seed" for its reconstruction. However, the Forerunner survivors suspected it would still take long before the network was truly healed.
Over 100,000 years later the Domain was accessed once again. In 2557, during their time on Requiem, Master Chief Petty Officer John-117 and Cortana were able to partially access the Domain via local terminal nodes, learning more about the history of the Didact and the Librarian. However, Cortana mentioned that the terminal was "caught in a loop" trying to access the Domain, leaving it unclear whether the fragments actually originated from the Domain itself.
After being rescued from Geranos-a in 2557, 343 Guilty Spark attempted to access the Domain, but was blocked by Catalog who revealed he had also done the same to the Ur-Didact. Later, after finding a personality imprint of the Librarian, Spark requested that she send him to the Domain to be with his friends after he was talked out of bringing them back to life. The Librarian warned Spark that the only thing left of Riser and Vinnevra in the Domain were echoes, "experiences remembered through the eyes of my kind, nothing more." The Librarian stated that Spark was meant to not meant to dwell on the past and live in the halls of living memory and warned that in the Domain, "the bad lives alongside the good." The Librarian helped Spark to see that the friends he sought for 100,000 were amongst the Ace of Spades crew and Spark gave up his quest. Spark later admitted to Rion Forge that he had sought the Librarian to access the Domain and either bring his friends back as geas or to join them.
After her apparent destruction along with the Mantle's Approach, fragments of Cortana's personality found their way to the Domain, where its guardian, Warden Eternal, confronted her. During the two intelligences' subsequent conversation, Cortana managed to infiltrate the Domain, much to the Warden's surprise. This seemingly "cured" her rampancy and enabled her to subdue the Warden under her newfound power. Cortana became convinced that she and other artificial intelligences — the "Created" — should seize the Mantle of Responsibility and rule over their organic "Creators". To accomplish her mission, Cortana assumed control over the Forerunners' Guardian network via the Domain. In October 2558, Cortana launched the Guardians, which had lain buried under numerous planets for a hundred millennia. She contacted John-117 via a vision through the Domain, declaring that the "Domain [was] open" and that the "Reclamation [was] about to begin", prompting John to go AWOL in search of her, accompanied by Blue Team. Days later, Blue Team found their way into the Domain through an entry point on the Domain hub world of Genesis, though they were unable to stop Cortana from unleashing the Guardians on the galaxy.
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