From Halopedia, the Halo wiki
Devils... Monsters... is the thirteenth piece included in the Halo: Original Soundtrack.
The name of the piece is a strong reference to the Flood, as it plays in the Library and for many of their other appearances. It repeats several notes many times, with carefully placed variations designed to surprise the listener. Much of the sounds are electronically-generated, and violins are incorporated at 0:12. An odd drum beat remains throughout the piece, reinforcing the eerie feeling of the music played. The piece is reorchestrated into Demons and Heretics in the Halo: Combat Evolved Anniversary Original Soundtrack.
It can be heard: