From Halopedia, the Halo wiki
Towards the end of the Covenant War, Sig Raan resided in a secret facility on the planet Otraak, a facility operated by Kig-Yar and Unggoy. Raan enjoyed a successful research career, helping the empire and contributing to their cause through the Covenant War. Although Raan's work for the Covenant was a lucrative career for her, the San'Shyuum often interfered with her work imposing their holy writ on Raan and her research.
Interactions with the Banished
As the Banished grew in power, Decimus sought Raan in an effort to expand the Banished's technology and fleets. Arriving at Otraak, Decimus and his forces were initially attacked by Raan's Unggoy guards, until she had them stand down upon recognizing Decimus. Raan allowed Decimus and his Jiralhanae into her facility, where Decimus offered her a place in the Banished, promising great awards from Atriox.
Instead, Raan presented her latest invention to the Jiralhanae, a device that mimicked the sonic and pheromonal signals of a Yanme'e queen, allowing its user to control dozens of Yanme'e drones. Having a Yanme'e carry her into the air, Raan blocked communications between her facility and the Banished cruiser over Otraak, and ordered her Yanme'e to kill Decimus and his Jiralhanae as traitors to the Covenant. The Jiralhanae were initially overwhelmed until Atriox, having predicted Raan's betrayal, joined the fight, aiding his forces in killing the Yanme'e. While Raan attempted to escape, Decimus killed the Yanme'e carrying her and the T'vaoan fell to her death.
Rather than use the Yanme'e controlling device, Atriox destroyed it, wanting those within the Banished to join willingly rather than by force.
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