Dread Intrusion
From Halopedia, the Halo wiki
Dread Intrusion is a song included in the Halo 3: Original Soundtrack. It plays during the level Floodgate.
Note: Around the middle/end of the song, it plays a remixed version of the song Ghosts of Reach. The version from Ghosts of Reach is heard around the 1:10 mark.
It runs for 5:28.
