From Halopedia, the Halo wiki
Greatest Journey is the twenty-third track (Disc 2 Track 8) included in Halo 3: Original Soundtrack, and plays in the Halo 3 campaign level Halo. It is composed of Greatest Journey (0:00-1:21), Flood Rising (1:21-1:40), and Halo Finale (1:40-4:52).
Greatest Journey (1:21) begins with a reprise of the vocals from Halo Reborn, featuring the mournful vocals singing the melody of Under Cover of Night (from Halo: Original Soundtrack, but with additional accompaniments of somber melody played by low strings. The low strings play variations of the Halo Theme, swelling to an end. This first section plays in the cutscene after John-117 has battled 343 Guilty Spark and activates Installation 08. It transitions into low synth notes playing in an eerie ambiance. This plays as John-117 and Thel 'Vadam leave the control room.
Flood Rising (1:19) is a short transition featuring digital effects playing in ambiance, fading in and taking over as the main focus. This piece plays when John-117 and 'Vadam enters the hallways leading away from the control room.
Halo Finale (3:42) is a full rearrangement of the third section of Truth and Reconciliation Suite (itself a variation of the Halo Theme), featuring a larger focus of orchestral sounds and ending with a brief piano solo. The piece is not a complete recreation of the music heard during the Warthog run sequence of the level. The in-game version begins with the male vocal parts from the original Halo Theme (before the final reprise of the Gregorian chant) from Halo: Original Soundtrack. The in-game version also features an extended version of Ancient Dust (from the earlier track To Kill A Demon) in the beginning as well as a faint Gregorian chant near the climax. Despite the minor changes, the piece plays in full in the final segment of the Warthog run, up until the final cutscene in which the UNSC Forward Unto Dawn escapes Installation 08.
The name Greatest Journey alludes to the central goal of the Covenant religion, commonly known as the "Great Journey".