From Halopedia, the Halo wiki
X-06 is the first exclusive track in the Halo Wars: Original Soundtrack Bonus DVD.
The track opens with strings and woodwinds playing a flowing melody. A solo clarinet take over, playing a somber rendition of the first two phrases of the Gregorian chant from the Halo Theme. Strings and percussion take over with a heavy melody building up to a climatic ending.