|"A Contest of Aeons"|
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- “A song used in aeon battles in FFX. It has a breathless quality that does not relent as the wondrous tune and lavish strength pour down upon the player...”
- —Theatrhythm Final Fantasy description
"A Contest of Aeons" (召喚獣バトル, Shōkanjū Batoru?) also known as, "Summoned Beast Battle" is the boss theme that plays during the final battle sequence against Yuna's Aeons which have been possessed by Yu Yevon in Final Fantasy X. In the International and PAL versions, this track also plays when the player encounters Penance.
This track is composed by Junya Nakano. The song begins with the melody of the Hymn of the Fayth, then an original part is led by the harp. The final section contains the motive of "Enemy Attack", the generic boss theme by the same composer. It is the seventeenth track on disc 4 of the game's original soundtrack.
An arrangement of this battle theme by Takeharu Ishimoto appears as the 42nd track of Disc 1 in the original soundtrack. It acts as the final boss theme for the duel between Yuna and the Emperor. It is entitled "Summoned Beast Battle".
An arrangement of "A Contest of Aeons" is included in this crossover between Final Fantasy and Dragon Quest for the PlayStation Portable. It is very similar to its original version, but its development was remixed.