The Halo Theme is the staple music of the Halo series.
The Halo Theme was composed over the course of three days in the summer of 1999 by Martin O'Donnell. O'Donnell was asked by Joseph Staten to provide a soundtrack for the Halo MacWorld premiere demo. Staten told O'Donnell that the theme needed to evoke the game's "ancient, epic and mysterious" ambiance. To best convey this impression, O'Donnell decided to incorporate a Gregorian chant, which has since become a staple of the Halo series in its various forms. He also drew inspiration from the Beatles' song "Yesterday", which helped him create the core melody. O'Donnell has stated that he laid down the core melody of the theme in about half an hour. The theme was recorded with an orchestra and a choir one day prior to the MacWorld presentation.
Halo: Combat Evolved
In its original incarnation, first heard in the Halo announcement trailer, it placed heavy emphasis on deep, powerful drums and fast paced strings, mainly the violin.
The track is heard most prominently at the start of the level The Silent Cartographer, during the beach landing and during the final Warthog run segment of The Maw. The chant at the beginning of the piece was used for the main menu screen in-game.
For Halo 2, the theme was remixed into the MJOLNIR Mix. The basics of the theme remain the same, but it features electric guitar overlays by former Whitesnake guitarist Steve Vai. Halo 2 uses the track in its original form in certain parts, such as at the beginning section of the level Metropolis. The MJOLNIR mix is played at the end of Metropolis as John-117 boards the Scarab, and later during the end credits. It is the first track on the Halo 2 Soundtrack Volume 1.
The theme was again revised for Halo 3, arranged into the song One Final Effort, this time recorded with a live orchestra instead of synthesized strings with the grand piano as its main instrument. The version used in Halo 3 finishes with the ending used at the end of The Maw, as opposed to the reprise of the opening chant used in the original theme and the MJOLNIR Mix. Occasionally, 343 Guilty Spark can be heard humming it with the IWHBYD skull activated in campaign.
The Halo Theme was reprised as the song Sacrifice. The chant portion of the theme, now referred in the Halo 4 credits as "Halo Cantorum", is heard when Installation 03 is revealed in the level Composer. The main notes of the Halo Theme play in the theme 117 when John neutralizes the external shield generators on Mantle's Approach.
Notes that ambiguously sound similar to the main chorus play subtly when Dr. Halsey tells her interrogator not to underestimate the Spartans.
Halo 5: Guardians
The Halo Theme returns in Halo 5: Guardians, rearranged as the The Trials with more focus on electronic instrumentation. It's also featured as part of the soundtrack in Halo Canticles. It was recorded at Abbey Road
The Halo theme was featured in the Halo Infinite - E3 2019 - Discover Hope Trailer implying its return in the sixth mainline Halo game.
The Halo Theme does not appear in Halo 3: ODST. While stated before release that the Theme would not appear in Halo: Reach, several tracks in the soundtrack borrow motifs from it. For example, Immemorial from Overture takes three bars from the beginning of the chant, while Unreconciled from Tip of the Spear takes the drumbeat and uses both the beginning of the chant and the main melody.
Two variants of the Halo Theme are featured in the Halo 2 Anniversary Original Soundtrack. The Halo Theme Gungnir Mix is a remixed version of the MJOLNIR Mix from Halo 2, and the Halo Theme Scorpion Mix is a percussion heavy version of the main theme.
A partial remix of the track appears in the Halo Legends Original Soundtrack. This remix plays during Halo Legends episode Origins during the scenes depicting the escalation of the Human-Covenant War.