Dread Intrusion
From Halopedia, the Halo wiki
Dread Intrusion is a piece included in the Halo 3: Original Soundtrack. It plays during the level Floodgate, and includes the tracks Dread Intrusion, Black Tide, and Old Friend.
The middle part of the piece (Black Tide) is a remixed version of the song Ghosts of Reach from the Halo 2: Original Soundtrack.
It runs for 5 minutes and 25 seconds.
At around 0:50, a reversed message can be heard. The voice sounds and echoes similar to the Gravemind. It says:
