Sin was created by Yu Yevon during the Machina War, years before the events of Final Fantasy X, as a means of protecting the dream Zanarkand he created to preserve the real city of Zanarkand, which was defeated by Bevelle, the centre of the Yevon religion. At first, Yevon (not to be mistaken with the in-game religion of the same name) controlled Sin, but the strain of controlling it destroyed his mind, leaving the creature's instincts to control it. It started destroying cities across the fictional world of Spira indiscriminately, starting with Zanarkand itself. Yunalesca, the daughter of Yevon, created a powerful creature known as the Final Aeon by sacrificing her husband Zaon. Zaon destroyed Sin but was possessed by the spirit of Yevon, slowly being transformed from the Final Aeon to a new Sin. A cycle started where summoners would go on a pilgrimage to the ruins of Zanarkand with their guardians, where the spirit of Yunalesca would create a new Final Aeon from the chosen sacrifice, which the summoner would then use to defeat Sin, which would be possessed and transformed into a new Sin, only for the cycle to repeat itself.
This cycle would continue until Tidus, an escaped spirit fro the dream Zanarkand, ended up in the real Spira. Becoming one of the guardians of Yuna, he would accompany her to Zanarkand, where all six of her guardians and Yuna herself would fight Yunalesca. After defeating Yunalesca, the party would face Sin itself above Bevelle on the Fahrenheit, an airship Yuna's uncle Cid dragged up from the bottom of the ocean. After defeating Sin, the seven of them entered the creature itself, where they fought Tidus' father Jecht, the chosen sacrifice for the Final Aeon of Braska, Yuna's father. With the defeat of Sin's life, Yu Yevon's spirit appeared to fight off the intruders and take a new body. After possessing each of Yuna's aeons, the spirit was defeated in combat with the party, after which Sin was truly dead.
Sin has a list of gravity-based abilities, the most powerful of which is the Giga-Graviton, which can create movement in Spira's tectonic plates that can be seen from space. In other words, it could effectively rip the planet's continents in half with one attack. It could also fly or swim, and had the ability to create creatures known as Sinspawn. The variants of Sinspawn are: