Devils... Monsters...

200px|right Devils... Monsters... is a piece included in the Halo: Original Soundtrack. It runs for 1 minute and 30 seconds. It is The Library's main theme and is most often heard in this level. It is also played when escaping the structure during the level 343 Guilty Spark through a blood-stained elevator shaft and during the Banshee run in Two Betrayals. It is played in the middle part of "The Maw" when you enter the cryo area, and again if you stay too long in the armory.

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 0:11. An odd drum beat remains throughout the piece, reinforcing the eerie feeling of the music played. Parts of this song are used in later games in the Halo series, most notably for the Halo 3 battle with 343 Guilty Spark in the song Halo Reborn during the level Halo.

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.