The party enters the Zanarkand Ruins where the pyreflies are so numerous the party witness visions of past summoners who came through to defeat Sin and see a vision of a young Seymour, whose mother gives up her life to become the fayth of Anima.
They meet Lady Yunalesca in Zanarkand Dome, now an unsent after her battle with Sin a thousand years ago, and learn that to create the Final Aeon a guardian must be sacrificed as its fayth. They glimpse into the past into Braska’s pilgrimage where Jecht became the fayth for the Final Aeon. The Final Aeon can only defeat Sin for a short time, for after the old Sin is destroyed the Final Aeon is transformed into a new Sin, and thus whoever becomes the Final Aeon will later become Sin, continuing the spiral of death.
Yuna refuses to partake in what she calls a “false tradition” as she does not wish to sacrifice any of her guardians. Yunalesca believes that without faith in the Final Summoning, and the hope it’s meant to bring, Yuna will succumb to despair, and decides to kill her and free her from such a fate. The party sees a vision of young Auron from the past; distraught about Braska’s death and Jecht’s transformation, Auron went to confront Yunalesca, but was killed, making him an unsent.
In the present time Auron rallies his team to attack Yunalesca. Though Yunalesca is powerful the party prevails and destroys her, forever ending the practice of Final Summoning. Before she fades, Yunalesca mentions that “even if there was another way… even if you did destroy Sin… Yu Yevon the immortal will only create Sin anew”.