VS. Nightmare

VS Nightmare, as named by the Metroid Prime & Fusion Original Soundtracks, is a music theme from Metroid Fusion. It was composed by Minako Hamano and Akira Fujiwara.

Description
As its name indicates, VS Nightmare is played during the boss fight against Nightmare in Sector 5. It is unique to this specific boss battle and has never been heard again in Fusion or any other Metroid game.

Contrary to what one might expect from a boss theme for a nightmarish monster, VS Nightmare is quiet and subtle, building up an unsettling creepiness rather than trying to actively scare the listener. It consists of a repeating rising sequence of bass notes; a long glissando that rises in pitch; two piano notes; and a series of high-pitched notes similar to those heard in Sector 5 (ARC) Low-Temperature Zone Area. This phrase repeats numerous times throughout the song, played in varying keys to give the song some progression. It can be heard here:

Trivia

 * Although Nightmare has two more boss fights in Metroid: Other M, neither battle features its titular theme. It shares its first theme with Goyagma and its second theme with Reo. In contrast with VS Nightmare, both of these themes go for the typical scary music style.