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“With the Covenant defeated the UNSC exodus from Draetheus-V seemed complete. The first sign that something else was happening came in reports of a low frequency static. UNSC analysts quickly identified the noise as a looping string of Forerunner code. Something was still active on Draetheus-V's moon, and it seemed to be calling out for attention. Spartan Sarah Palmer returned to the surface of X50, Draetheus-V's moon to discover the UNSC was not the only one answering the bizarre Forerunner call. The Covenant had returned as well. They believed one of their gods was speaking to them and they were adament that the humans would not intercept the message.”
— Mission summary