Yunalesca used her husband, Lord Zaon, as a fayth to summon her Final Aeon. After that, as an unsent, she greets the summoners who make it to Zanarkand and explains that they must sacrifice one of their guardians to become the fayth for the Final Summoning.
For a millennium this tradition went unchallenged, until the arrival of Lord Braska and his guardians, Jecht and Auron. Though Auron had a change of heart and was no longer willing to witness the sacrifice of his friends, Jecht nonetheless chose to assist Lord Braska and become his Final Aeon. The rest of Spira celebrated the Calm but Auron was devastated by the senseless loss of his two friends. Returning to Zanarkand, he confronted Yunalesca, learned of the inevitable return of Sin after the Final Summoning, and attacked her in a fit of rage. Auron was struck down easily, sustaining a massive head wound in the process. Remembering his promise to protect Braska's young daughter, Auron then crawled down Mt. Gagazet but died on the outskirts of Bevelle.
Ten years later, Lord Braska's daughter Yuna (who was named after Yunalesca herself) decided to follow in her father's footsteps and become a summoner. Sir Auron, now an unsent, joined Yuna's retinue at Luca and accompanied her along with the rest of her guardians to Yunalesca's chamber in Zanarkand. Upon learning of the futility of the pilgrimage, Yuna refused to support Yevon's false traditions in spite of Yunalesca's remonstrances that it would bring hope to the people of Spira. Yunalesca then decided to save Yuna from her own sorrow by granting her the peace of death; the guardians fought back, and Yunalesca was seemingly killed.
Broken by her defeat (followed shortly by that of her father), Yunalesca's soul traveled to join that of her husband in the Via Infinito under Bevelle. Over the next two years, a now "purposeless" Yunalesca became a giant basilisk-type Fiend Chac, finally put to rest by YRP.
Yunalesca was something of an enigma. Whether she was a true villain, or simply misguided is a matter for debate as her motivations are never fully explained.
Maechen speculates that her committment to the peverse status quo might be interpreted as a way of honouring two opposing forces to which she felt particular loyalty - Yu Yevon, her father, and the people of Spira. "It was to quell his wrath that they revered him... and so were born the temples of Yevon... A fair trade, she defeats Sin in exchange for her lord father's honor."
Though there is no way to know for sure, it is worth noting that several other characters in Final Fantasy X did the "wrong" thing for the "right" reasons. Probably the most notable was Seymour Guado, who thought that the only way to save Spira was to destroy it.
The party fights Yunalesca in the Zanarkand Ruins. She has three forms and often casts zombie status on the party. Yunalesca is a notoriously hard battle, and is regarded as the hardest battle in the main storyline.
|Tidus - Auron - Rikku - Wakka - Lulu - Yuna - Kimahri|
|Temporary playable characters|
|Jyscal Guado - Kelk Ronso - Wen Kinoc - Yo Mika|
|Chappu - Clasko - Elma - Gatta - Lucil - Luzzu - Lord Mi'ihen|
|Belgemine - Braska - Dona - Gandof - Isaaru - Ohalland - Yocun - Yunalesca - Zuke|
|Barthello - Biran Ronso - Borra - Brother - Calli - Cid - Ginnem - Jecht - Maechen - Maroda - O'aka XXIII - Omega - Pacce - Rin - Shelinda - Tidus' Mother - Tromell - Wantz - Yenke Ronso - Yu Yevon - Zaon|
|Baaj Temple - Besaid Island - Kilika Island - Luca - Mi'ihen Highroad - Mushroom Rock Road - Djose - Moonflow - Guadosalam - Farplane - Thunder Plains - Macalania - Bikanel Island - Bevelle - Calm Lands - Monster Arena - Cavern of the Stolen Fayth - Remiem Temple - Mount Gagazet - Zanarkand - Zanarkand Dome - Omega Ruins|
|Al Bhed - Guado - Hypello - Ronso|
|Aeon - Blitzball - Butterfly Hunt - Calm - Celestial Weapon - Chamber of the Fayth - Chocobo Knights - Cloister of Trials - Eternal Calm - Fahrenheit - Fayth - Fiend - Final Aeon - Guardian - High Summoner - Hover - Jecht Shot - Machina - Maester - Overdrive - Overdrive Mode - Overkill - Pilgrimage - Pyrefly - Rin's Travel Agency - S.S. Liki - S.S. Winno - Sending - Shoopuf - Sin - Sphere - Sphere Grid - Summoner - Unsent - Warrior Monks - Yevon|
|Original Soundtrack - Promo CD - Suteki da ne - Vocal Collection - Feel/Go Dream: Yuna & Tidus - Piano Collections - Official Soundtrack
|Abilities - Armor - Auto Abilities - Black Magic - Characters - Chronology - Enemies - Enemy Abilities - Items - Locations - Ronso Rage - Translations - Walkthrough - Wallpapers - Weapons - White Magic|