Sin

Every ten years, a gargantuan beast appears for one purpose: to bring annihilation to technology. It was given the name 'Sin' by the people of Spira because they believed the best came upon them from their sins due to their technology use which started the Machina war between Bevelle and Zanarkand. For numerous generations, there was only one way to defeat Sin.  A Summoner must go on a pilgrimage to the ancient city of Zanarkand. The Summoner must offer one of her guardians to Lady Yunalesca, the first summoner to defeat Sin.  Lady Yunalesca would forge the guardian into a powerful Aeon known as the Final Aeon. The Final Aeon would have enough power to defeat Sin but the outer shell of Sin would be destroyed. Yu Yevon, the creator of sin and Lady Yunalesca's father,  would posses the Final Aeon to create a new sin that would have the soul of the sacrificed guardian in its core.  The period known as the Calm is the time where " baby Sin " would grow and not threaten the people of Spira.

Another risk is the Summoner dying in the process, but not by the Final Aeon attacking the summoner, but by the Summoner must give her or his life energy to create the Final Aeon's into being. Another way the Summoner could die is by the strong mind link between the Summoner and the Final Aeon share instantly results in the death of the Summoner.

In Final Fantasy X, Yuna and her guardians go on a pilgrimage. They uncover the plans of the Master of Yevon, Seymour Guado. Seymour's plan was to marry Yuna so when she reached Zanarkand she would offer him to Lady Yunalesca where he would become the Final Aeon and eventually Sin. Seymour's desire to end the suffering of Spira by annihilating all the people of Spira was stopped by Yuna and her guardians such as Tidus and Auron. When Yuna and her guardians reached Zanarkand, Lady Yunalesca revealed the true nature of the Final Summoning. Yuna is shocked that she has to sacrifice one of her guardians but with Tidus's encouragement the party decides to attack Sin at its core. Once they defeat their possessed Aeons and Tidus's father Jecht, they come to a confrontation to its core, Yu Yevon. Since they have defeated his possessed Aeons, Yu Yevon is destroyed by Braska's Final Aeon, Jecht. Once Yu Yevon is destroyed, the consequence is Tidus broke Yu Yevon's hold on fayth, souls that are sealed in the status of dreaming, Tidus begins to disappear. The result forces the fayth to wake up into nothingness. Before Tidus leaves to the Farplane, Yuna tries to hold him and says that she loves him and lets him go. Since Sin is no more, a new time period has been created for Spira, eternal Calm, further explored in the sequel, Final Fantasy X-2.