Keep What You Steal
From Halopedia, the Halo wiki
Keep What You Steal is the twenty-first track (Disc 2 Track 6) included in Halo 3: Original Soundtrack, and plays in the Halo 3 campaign level Cortana. It is composed of Keep What You Steal (0:00-1:12) and Escape (1:12-2:36).
Keep What You Steal (1:12) starts with a single high strings note, followed by a piano variation of the five-note E Dorian melody (from the track Finish the Fight). Strings then play the melody of the Gregorian chant from the Halo Theme. The melody begins to swell, then quiets to a minor key to transition to the next piece. The piece plays in the cutscene before the "Nor Hell A Fury..." chapter, in which John-117 reunites with Cortana. The piece was played in full reprise in The Package (from Halo: Reach Original Soundtrack).
Escape (1:24) begins with the strings playing a single note, but drums in the background gradually grow louder and louder until they block out the strings. The strings take over and begin a discordant melody, which continues to grow louder before fading out in the end with an echo. The piece plays when John-117 destroys the reactors of High Charity and escapes the Flood hive, then continues into the ending cutscene in which John-117, Cortana, and Thel 'Vadam escape High Charity on a D77H-TCI Pelican.
The name of the track is derived from Cortana's quote from the campaign level The Covenant: "I'm a thief... but I keep what I steal."