Final Fantasy X-1
For a thousand years, the world of Spira was threatened by the ever-present menace of a huge, supernatural leviathan that destroyed their cities and killed their people. They believed it to be the punishment for mankind's reliance on machina (machines). They called it "Sin".
Most of Spira lived under the rule of the Church of Yevon, abiding by the teachings in the hope of atoning. Foremost amongst Spira's struggle against Sin were the Summoners. A Summoner would go with his or her Guardians on a Pilgrimage to every temple in Spira, praying to the Fayth to receive its Aeon. Preparing the mind and body to receive the Final Aeon.
But in truth, Sin WAS Yevon. A Summoner from a thousand years ago, he grieved the loss of his home of Zanarkand during its war with Bevelle, who destroyed Zanarkand with machina weapons. Mad with grief, he took the souls of the surviving people of the city and turned them into Fayth. He used them to summon an idyllic Dream of Zanarkand. And also to summon "armor" for himself, the creature which would come to be known as Sin. He immediately lost control, and it began destroying Spira.
Before this, he gave instructions to his daughter Yunalesca how to defeat Sin temporarily. Using these instructions, she turned her husband Zaon into a Fayth and used the resultant Aeon to fight Sin. This was the Final Summoning. However, not only did she die in the battle, but her husband became the Fayth of the new Sin. Yunalesca stayed on as an Unsent -- a ghost given physical form -- waiting in the temple in the destroyed Zanarkand. Her purpose? To transform one of a Summoner's Guardians into a Fayth so that the Summoner could receive the Final Aeon. The Final Aeon would then be summoned against Sin, Sin would fall, and Yevon would possess the Fayth of the Final Aeon, transforming it into the new Sin.
The cycle continued for a thousand years. Until a young woman named Yuna, whose father Braska had been a Summoner before her -- High Summoner, in fact (the title given to a Summoner who used the Final Aeon to defeat Sin) -- started her Pilgrimage. With a group of ragtag Guardians -- including one of her father's Guardians and the son of his other Guardian -- she learned the awful truth behind Sin and Yevon's spiral of death. And she made the decision to stand strong, and look for a permanent solution.
Through her loss of faith, and both physical and emotional trials she'd never dreamed of, she remained strong. And eventually she and her Guardians ended the menace of Sin for good. But it was not without a price. The Fayth needed to finally die, and without them, Dream Zanarkand could not exist. The dream faded, the son of her father's Final Aeon faded as well, her father's other Guardian (who was also Unsent) faded away too.
But for better or worse, Spira was finally free from the menace of Sin. Now the task turned to rebuilding their shattered world. And the shattered bonds between the people of Spira. Only together would they overcome the aftermath of Sin.