Halo 3 Soundtrack - One Final Effort
(if my memory serves me correct it's the song that plays while you battle it out with the two scarabs before you enter the citadel in the level, 'The Covenant' and while you proceed to the Forward unto Dawn in the last level, 'Halo' (it also plays during the 'beginning' of the final cutscene (well, second last if you include after the credits)