This is the Hour
From Halopedia, the Halo wiki
This is the Hour is a song included in the Halo 3: Original Soundtrack. It is one of the most recognizable themes in the entire game.
The song plays during the ending cutscene of The Storm.
The very beginning is mainly choir, followed by the well-known piano motif. This is then repeated by the entire orchestra. The song swells to a dramatic climax, then abruptly stops.
It runs for 2:06.