"Mantis' Hymn" is a recurring song in the Metal Gear series, and is the main theme of FOXHOUND member Psycho Mantis (and in the case of Metal Gear Solid 4: Guns of the Patriots, Screaming Mantis). It is a dark, somewhat repetitive song with chorus hums, meant to emphasize the ominous presence of the FOXHOUND psychic, Psycho Mantis. Although the song is thought to be mainly used out-of-universe, it was given two acknowledgements in-universe, when Naomi Hunter asks Solid Snake whether he hears the song and when Naomi reveals that Meryl Silverburgh is being controlled by Psycho Mantis due to the song being his mind control music.
It first appeared in Metal Gear Solid, playing when Solid Snake first encounters Mantis. It later reappears in Metal Gear Solid: The Twin Snakes, a remake of the former game, although it is remixed somewhat, also giving rumbling sounds reminiscient of thunder, and also having a slight shriek before the characteristic chorus chant. Portions of the song are also incorporated into his battle theme in the latter game, which replaced "Encounter."
In Metal Gear Solid 4, it reappeared again, both in its original format (played when Psycho Mantis appears in spectral form, with some hints to the remixed version from The Twin Snakes as Mantis is being banished to the spirit realm by The Sorrow), and in a remixed format where a female chorus is humming the characteristic vocalizations (as well as a slight low-pitched and drawn out shriek after the first stanza), and also having a more fast-paced version, which is played during Screaming Mantis's first appearance and during the fight with her when using the Psycho Mantis doll against her, respectively, as a callback to Psycho Mantis. Additionally the track could be played on the iPod item. The Metal Gear Solid 4 version of the song was also playable as an optional track in Metal Gear Online.