"A Contest of Aeons", also known as "Summoned Beast Battle"(召喚獣バトル, Shōkanjū Batoru?), is the boss theme that plays during the final battle sequence against aeons in Final Fantasy X. In the International, PAL and HD Remaster versions, this track also plays when the player encounters Penance.
This track is composed by Junya Nakano. The theme 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 original soundtrack.
The theme plays when Yu Yevon possesses Yuna's aeons. They turn to a dark hue and turn on the party, and must be felled one by one. The sequence is longer if Yuna has obtained the secret aeons as well.