This is the Hour
From Halopedia, the Halo wiki
Just as the ending cutscene heavily resembles the Halo 3 announcement trailer, This is the Hour is nearly identical to the track Finish the Fight, the music played in the trailer. The piece begins with a soft drumroll, followed by strings and choir building up the melody to a piano solo of the five-note E Dorian melody from Finish the Fight. This is then repeated by the entire orchestra. Percussion enters, and the strings swells to a dramatic climax with the Halo Theme triplet rhythm. The melody reaches with high strings, but stops with a single high strings note. Ambient strings then take over till the end.