On a Pale Horse
From Halopedia, the Halo wiki
On a Pale Horse is the nineteenth soundtrack included in the Halo: Original Soundtrack.
The soundtrack begins with driving staccato notes from the high strings joined by pizzicato from the low strings in what can be guessed is an alternating time signature between 6/4 and 4/4 time. At various times, sections of the high strings voice themes and variations on the "monk chant", eventually developing a unique melody. At about a minute into the soundtrack, the strings start a sort of "call - answer" theme again divided by low and high strings, then reverts to the staccato theme this time voiced in the lower register while the higher register plays slow chords while they crescendo to the end of the soundtrack.
It can be heard:
It is remastered into the following:
The name "On a Pale Horse" seems to be a reference to the Four Horsemen of the Apocalypse in the book of Revelation; specifically, it refers to the fourth horseman, who'll ride in on a pale horse, and represents death. It is probable that this fourth horseman is meant to be John-117, who will slay the Covenant. Incidentally, the first chapter of the level The Maw is titled "And the Horse You Rode In On", though it is most likely a reference to UNSC Pillar of Autumn.