From Halopedia, the Halo wiki
After Draetheus V was disintegrated by X50, large parts of the planet crashed into the moon and revealed a Forerunner structure. This was discovered by the UNSC following their return to the moon in search for the source of an unknown signal emanating from below X50's surface. The structure led to the Cistern and Spartan Sarah Palmer entered. She encountered numerous Stealth Sangheili, but she was able to defeat them. As Palmer and accompanying engineers moved deeper into the Cistern, the engineering equipment began to become unreliable and the signal started interfering with anything that could pick it up. However, Spartan Palmer's MJOLNIR armor was able to withstand the interference.
At the end of the cave system, she encountered Spartan Edward Davis' remains, which had been absorbed into a mysterious device. The signal turned out to be the final impressions of Davis' consciousness, before his death earlier in the battle. Palmer shut the signal down and both her and the device housing Davis' remains were airlifted via a Pelican.
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