Yunalesca is a daughter of Yu Yevon, who created a technique called "The Final Summoning", the only way Sin's hard armor could be pierced. In the backstory, she was the first person who defeat Sin, using "The Final Summoning" technique, she sacrificing her husband, Lord Zaon. She died in the battle, but Yunalesca remained as an unsent to instruct her successors in their pilgrimage to defeat Sin's ever-regenerating incarnations. When the party refuses to sacrifice a person in the Final Summoning, Yunalesca fights the party but she defeated.
Yu Yevon was a peerless summoner live one thousand years before the events of Final Fantasy X, He is the ruler of Zanarkand. He is responsible for much of Spira's circumstances, first taking the lives of his own people to create "Dream Zanarkand," and then drawing on millions of pyreflies with Gravity spells to create Sin. Yu Yevon rebirths Sin by possessing the Final Aeon used to defeat it (this is how Jecht becomes Sin). The survivors of the events surrounding Sin's genesis adopted the Teachings of Yevon and built a religion around praising Yevon in the hopes that they may quell his wrath
The summoner who trains his/her aeon summoning abilities could call the Final Aeon at the end of her/his pilgrimage which the Guardians who travel with her/him to ensure her/his safety. The Final Aeon is an ultimate summon created from a guardian of the summoner itslef, who has a close bond with him/her. The guardian is killed and turned into a fayth, The fayth created from the Guardian then controls the Aeon. Summoning the Final Aeon destroys Sin, but Yevon would endure and merge with the Final Aeon to craft a new Sin from it. The summoner, after defeating Sin with the Final Aeon, loses his/her life to it because of Yevon merging with the Aeon and killing the summoner in the process (This was happen to Lord Braska). Yunalesca proclaims that this rite can defeat Sin for a time and give Spira hope for a short while, but unless the world embraces the teachings of Yevon and atones for their sins, Sin will never disappear completely.
Bevelle was still terrified of Yevon and Sin, and in order to appease Yevon, St. Bevelle began to follow his teachings. The teachings mainly revolve around the Final Summoning and the ban of machina, as it was Yevon's wish that there should never again be another machina war. St. Bevelle began to set up temples throughout Spira, and in each temple is a Fayth. To govern these laws, the Order of Yevon came to be, with four Maesters of Yevon at the top, a High Priest for each temple, and many other priests, monks and nuns.A Zanarkandian song which later becomes known as the Hymn of the Fayth is taken up by those who defy Bevelle, including the fayth inside the temples. At first Bevelle prohibits the use of the song. Al Bhed then pick it up, change it into a song against Yevon in which Bevelle couldn't do anything to stop them. Then a new story started spreading from Bevelle about how the song was sung to soothe the souls of the dead in which they made the song into scripture and it spread throughout all of Spira in connection with Yevonism as a holy song.
In the end, Yuna and her guardians use her aeons to weaken the disembodied spirit of Yu Yevon, allowing them to destroy him and bring an end to Sin and the spiral of death that had consumed Spira for 1,000 years. The party will battling with Yu Yevon at the final boss battle, he attacking only with Gravija, which cannot kill the party member. So there's no way to lose the battle except you attacking your own party member and kill yourself at the end.