From Halopedia, the Halo wiki
Within 72 hours after October 17, 2558, a Forerunner Guardian Custode was activated, causing large-scale destruction to a dense urban area. In the same window of time, Ursa IV, Oban, Adelaide and Queen Anne VI were also affected by similar events.
Before the site could be secured by the Office of Naval Intelligence, the New Colonial Alliance took control of it. Not willing to let the Guardian fall into the NCA's hands, ONI tasked the operative Commander Maya Sankar to go undercover as the insurrectionist leader "FERO" and gather information by infiltrating the NCA, with the help of the AI Black-Box.
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