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Invariant Bias was commissioned over a hundred millennia ago to guard the Absolute Record, until its new custodian would arrive, commissioned by the Librarian to carry out the Reclamation. However, after seeing the betrayal of Mendicant Bias when he turned allegiance to the Flood, the custodian grew suspicious about who it should trust with the Record's power.
In mid-2558, the Absolute Record was visited for the first time in ages, by the CAS-class assault carrier Song of Retribution, the flagship of Jul 'Mdama and Catherine Halsey. Unknown to them, Spartans Sarah Palmer, Gabriel Thorne, and Holly Tanaka along with scientist Henry Glassman had stowed away aboard the vessel, the ancilla sensing the presence of the hidden humans. 'Mdama and Halsey met the ancilla with an enormous warrior escort, many of whom were uncomfortable with the numerous Sentinels inspecting them. When Halsey demanded the ancilla open the Record to her, it refused and instead summoned the other humans of the Retribution.
Despite Halsey having been given the Janus Key by the Librarian, the device that would unlock the Record, the ancilla still felt it could not trust her as it did not know the circumstances of how Halsey got the Key. It thereby prepared to test the visitors to see who deserved to be the new custodian of the Record. At this point, some of Jul's Sangheili attacked the Sentinels, so the ancilla responded by opening fire and then evacuating most of the floor, killing all the Sangheili but Jul and leaving the humans on platforms.
The custodian gave the two remaining parties a hypothetical scenario, in which a sentient species (depicted by Yonhet) turned out to have bodies resistant to the Flood. Harvesting live specimens would give a thirty-five percent chance of a vaccine, while harvesting dead bodies offered willingly would have a thirty-three percent chance. The UNSC group responded with the peaceful option while Halsey chose enslavement for the entire chance, the ancilla commending both. It then asked both parties whether their opponents being tested should be evacuated alive or killed. Before it could get an answer, Halsey hacked into the ancilla and took control of the facility herself.
In backup of its forceful hijack, a portion of the ancilla's memory was downloaded into a Sentinel, which requested the Spartans' aid in regaining control. It directed them to a maintenance chamber where the Spartans inputted codes to reactivate its primary shell. Once active, it captured Halsey and reprimanded her for being so hasty in controlling the Record. It then took the Janus Key and denied her further access to the Record.
Invariant Bias's shell resembles that of the fellow Contender-class Mendicant Bias, being shaped in a rough teardrop shape with three photoreceptor "eyes". These eyes are individually blue, green, and yellow. In the center of its shell is the Eld symbol. The custodian's shell can levitate and is roughly three times the size of a Spartan or Sangheili.
The ancilla controls an army of Aggressor Sentinels, and can also reshape the facility's walls and floors, including offensively to send victims to a fatal fall.
In case of sudden destruction or disabling, the ancilla's mind is programmed to enter a local Sentinel, which then can be restored to its original body through reactivation. On its own the process can take seven hours, but with the aid of helpers it can be sped up to roughly twenty minutes.
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