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Parasite Queen is the theme that plays during the battle with the boss of the same name in Metroid Prime. The theme somewhat resembles Mother Brain's theme in Super Metroid and Amorbis's theme in Metroid Prime 2: Echoes. It is available in the Soundtrack Gallery in all Wii versions of the game.
A remix is heard as the Morph Ball theme in Metroid Prime Hunters: First Hunt. A slightly-altered version is heard when battling the Fire and Arctic Spawns in Metroid Prime Hunters. The latter has an unused Sound Test title, Demon Spawn, in the internal "BGMSELECTLIST.DAT" file.
The original theme appears in Super Smash Bros. Brawl on the Frigate Orpheon stage, and on Pyrosphere in Super Smash Bros. for Wii U, both times titled Vs. Parasite Queen (vs. パラサイトクイーン vs. Parasite Queen ). In Adventure Mode: The Subspace Emissary, the theme serves as background music for the first part of The Research Facility (Part II). It is unlocked randomly by collecting CDs in Brawl. In Smash on Wii U, the song must be unlocked by recovering stat boots stolen by a Metroid in the Smash Tour mode. The original theme is also included on the Metroid Prime & Fusion Original Soundtracks, titled VS Parasite Queen.