Wake Me When You Need Me
From Halopedia, the Halo wiki
The track starts with a wind and strings melody variation of the Gregorian chant of the Halo Theme. The woodwinds play the main melody with echoes from the high strings. Low strings then join in and take on the main melody of Dust and Echoes (from Halo: Original Soundtrack), which played during the ending of Halo: Combat Evolved. The melody features variations of the Gregorian chant, briefly shifting to a minor key, only to be followed by a major chord at the end.
The name of the track is derived from the final words of John-117 in Halo 3: "Wake me... when you need me."