Vs. Ruins Test

"Vs. Ruins Test", identified as 21 in the Sound Test, is a theme from Metroid: Zero Mission.

Description
This theme plays while Samus Aran, armed with only her Zero Suit and Paralyzer, is completing the Ruins Test in Chozodia. Therefore, it serves as the boss theme for the God of War.

This boss theme is stylistically unique in the Metroid series, as the only song that predominantly features a pipe organ. With powerful low chords and rapidly-moving notes in the upper register, it comes across as evocative of classical and/or baroque music; the pipe organ's association with church also ties into how the spiritual and religious Chozo likely revered the God of War. The organ is occasionally accompanied by a deep choir, as is typical for Chozo music in Zero Mission and Metroid Prime.

The theme can be heard here: