Sacred Icon Suite 2
From Halopedia, the Halo wiki
The first part of the song consists of a heavily orchestrated version of Weight of Failure/(the final two minutes of) Destroyer's Invocation/Cortege (the Arbiter's motif) from the Sacred Icon Suite. The second part is a fully orchestrated and lively arrangement of Unforgotten/Never Forget, where a choir takes over the main melody, reinforced by a full orchestral arrangement, bringing the piece to its climax. The final few seconds feature the melody of Opening Volley.
The track can be heard in several places across the Halo franchise;