Combat Team X-Ray
From Halopedia, the Halo wiki
"X-Ray" was stationed aboard the UNSC Pillar of Autumn in 2552. Following its arrival at Installation 04 and the ensuing space battle, Covenant boarders would breach the Autumn. X-Ray was sent alongside other combat teams, including Tango, to port decks five and six. Eventually, Captain Jacob Keyes would initiate a general evacuation of the ship, ordering combat teams Alpha through November to evacuate. Combat Team Sierra through Combat Team Victor evacuated after them. Seeing as Victor comes before X-Ray in the Phonetic alphabet, it's unclear whether combat team X-Ray was part of a third phase of evacuation or were entirely defeated aboard the Autumn.
Whether of not Combat Team X-Ray made it to the surface of Installation 04 is unknown, though it is likely that any survivors were part of the battalion led by Major Antonio Silva later in the conflict. By the time of the destruction of the ringworld three days later, Combat Team X-Ray (along with all other local UNSC and Covenant forces) was killed.
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