From Halopedia, the Halo wiki
The track is a full reorchestration of the track Unforgotten (from Halo 2: Original Soundtrack), which is also its namesake, albeit featuring a more prominent choir accompaniment and crisper piano sounds. The melody begins with strings, but four keys lower, giving it a more solemn mood. The motif is then repeated with choir. Choir then takes the melody, with a piano accompaniment. The music then swells to a definite upturn, switching to a major key briefly before it switches back to a minor key. Piano then takes the melody and ends the piece with the main melody. With careful ears, a French Horn background melody can be heard.