From Halopedia, the Halo wiki
Black Tower is the seventeenth track (Disc 2 Track 2) included in Halo 3: Original Soundtrack, and plays in the Halo 3 campaign level The Covenant. It is composed of Black Tower (0:00-3:47) and Heroes Also Fall (3:47-6:03).
Black Tower (3:47) contains five sections. The first section begins with tribal percussion before ambient synth notes enter and then take over the piece completely. The percussion plays when John-117 and the Sangheili forces encounter a Jiralhanae pack in the back hallway of the third barrier tower. The ambient synth notes play when John-117 takes the elevator to the top of the third barrier tower. The second section of the piece features a reversed message over the synth notes. This section plays when John-117 battles the Flood forms that have breached the barrier tower. In the third section, low strings plays a somber melody, followed by the choir singing eerily in different variations. This section plays during the cutscene in which High charity arrives at Installation 00, and sends Flood dispersal pods to the area surrounding Installation 00 Citadel. In the fourth section, static ambiance take over until the transition into the next section. The fourth section plays when John-117 and the Sangheili forces return to the exterior of the third barrier tower. The fifth section begins with low strings playing in ambiance before choir enters singing in soft ambiance. It slowly builds suspense until the transition into the next piece. This piece plays in the first part of the cutscene before the chapter "Revelation", from John-117 and 'Vadam entering the Citadel up until Prophet of Truth says to Miranda Keyes, "You cannot hope to kill them all".
Heroes Also Fall
Heroes Also Fall (2:16) begins with high strings and choir in a rearrangement of Rue and Woe (from High Charity Suite from Halo 2: Original Soundtrack), featuring a larger focus on low strings and choir parts. The piece plays in the cutscene before the chapter "Revelation", from Keyes raising her M6G magnum at Avery Johnson up until the end of the cutscene, and the title alludes to Keyes' death.
Just like Destroyer's Invocation (from Mausoleum Suite in Halo 2: Original Soundtrack) and the earlier track Dread Intrusion, when Black Tower is played backwards, a voice similar to the Gravemind can be heard. The message references the second and third stanza of the poem The Hollow Men by T. S. Eliot.