From Halopedia, the Halo wiki
Devils... Monsters... is the thirteenth track in Halo: Original Soundtrack.
The name of the track 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 was arranged into Demons and Heretics in Halo: Combat Evolved Anniversary Original Soundtrack.
It can be heard: