From Halopedia, the Halo wiki
Abaddon was a powerful Precursor construct tasked to serve as the overseer of the Domain. By the latter years of the Forerunner civilization, Abaddon's nature had been forgotten by most Forerunners and it had passed into legend as the Organon, a fabled Precursor artifact of incredible power.
Abaddon was created by the Precursors in the ancient past as what Bornstellar Makes Eternal Lasting believed to be their equivalent of an artificial intelligence, albeit far more sophisticated than any Forerunner ancilla. Left behind by the Precursors after their passing, Abaddon evidently helped early Forerunners access the Domain. Discovered by ancient Forerunners in a remote part of the Milky Way galaxy, it would eventually come to be housed at the heart of Maethrillian, the ecumene's capital—specifically the Mysterium. Over time, the Ecumene Council would suppress public knowledge of Abaddon, and its true nature was eventually forgotten to all but the most high-ranked Forerunners while its name was corrupted to "the Organon", a Precursor relic rumored to be capable of activating all other Precursor artifacts. The Organon was considered to be the most sought-after Precursor artifact by Forerunner archaeologists.
Several years prior to 97,445 BCE, Bornstellar Makes Eternal Lasting sought the Organon on both Edom and Erde-Tyrene using a pair of human guides, Chakas and Riser, to lead him to its presumed resting place in the Djamonkin Crater. Instead he found the Cryptum of the Didact, who stated that the Organon did not exist, at least not in the form imagined by most Forerunners.
At the close of the Forerunner-Flood war, just minutes prior to the firing of the Halo Array, the Gravemind told the Librarian that the Organon was in fact the same entity as the Domain, a quantum information storage medium utilized by the Forerunners to a great extent, leading the Librarian to conclude that the Domain had been created by the Precursors. When the Halo Array was fired, the Domain and all knowledge within would be lost forever, along with all Precursor artifacts. The Librarian proceeded to send a message to the IsoDidact in which she pleaded him to find the Organon, which she stated was located at Maethrillian, and restore the Domain. The IsoDidact, now Bornstellar Makes Eternal Lasting again, would not view the Librarian's message until after the reintroduction, well over a century later. Shortly afterward he, Chant-to-Green, and several other surviving Forerunners embarked on a mission to Maethrillian in order to reactivate the Domain. They were accompanied on the expedition by former First Councilor Splendid-Dust-of-Ancient-Suns, who had been informed of some of the details about the Organon, though even he was not fully aware of its true nature.
At the capital world, Abaddon announced its true name to the Forerunners and attacked them with pseudo-organic constructs it had seemingly created based on Promethean Crawlers. Once the Forerunner party reached the Council amphitheater, Abaddon confronted them, representing itself with a massive holographic projection of a winged, androgynous Forerunner-like being. Twisted and embittered by the damage wrought upon the Domain by the Halos, Abaddon announced that the Forerunners were to face trial for their crimes against the Mantle. While Splendid Dust remained before Abaddon to accept his sentence out of guilt, Bornstellar, Chant-to-Green, and several others escaped deeper into the capital world where they discovered the physical location of Abaddon. Abaddon attempted to prevent the Forerunners from interacting with its housing, vaporizing two of them and attempting to immobilize the rest by locking their armor before it could pronounce its judgment on them. However, Growth-Through-Trial-of-Change managed to insert a deadbolt key into the device, sacrificing herself to serve as the template for the restored Domain. This act seemingly caused Abaddon to disappear and the Forerunner survivors were able to leave Maethrillian unhindered while the Domain began to heal.
As a Precursor intelligence, Abaddon was an incredibly powerful entity. It was capable of vaporizing physical beings at will, and could effortlessly interface with Forerunner technology, such as access their communications or personal armor. It could also create organic versions of mechanical constructs, like Promethean Crawlers or Aggressor Sentinels.
The physical appearance of Abaddon's avatar is described as massive in size, having a face neither male nor female, radiant, exquisitely terrifying, heartbreakingly wondrous. Its coloring is indigo, and it has massive wings as well.
Abaddon is Hebrew for "destroyer" or "destruction" and is also the name of an angel or demon in the Bible.
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