From Halopedia, the Halo wiki
In 2558, after Catherine Halsey was abducted by Jul 'Mdama, Fireteam Crimson was sent to "Cyclone" to find the source of communications Halsey had been relaying in this area. Dr. Truman sent Crimson the coordinates to a comm relay inside the structure. As Crimson approached the doors, they disintegrated, Truman felt the structure was responding to Crimson's presence. When they discovered that 'Mdama was rallying all his troops at a location known as "Librarian's Rest", Truman said she was unfamiliar with it. She said she could find the location of Librarian's Rest by continuing to trace the communications through the relay. When a sudden power spike occurred in the area, Roland pointed out their similarities, and she felt they were connected. When Crimson was suddenly attacked by a large Promethean force, some friendly turrets came online and opened fire on the Prometheans, Truman noticed that it was like someone was trying to help her decrypt the information. Suddenly Aggressor Sentinels appeared and aided Crimson against the Prometheans. After Crimson flips a breaker, the Sentinels started shooting the structure. It altered its shape and shot a beam from it. Truman said it was a data stream but too fast to analyze.
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