What Once was Lost
From Halopedia, the Halo wiki
What Once was Lost is the eleventh soundtrack piece included in the Halo: Original Soundtrack.
Played in levels featuring the Flood, the piece adds to the tension the player feels before and after discovering the Flood. It primarily uses swamp noises such as animal cries, with undertones to add to the suspense. The soundtrack itself creates much of tension to suit the dark, scary and eerie moods of the levels. The piece is remastered into Unfortunate Discovery in the Halo: Combat Evolved Anniversary Original Soundtrack.
The track can be heard:
The title of the piece likely alludes to the Flood and signifies its return.