What is Sin from Final Fantasy X?

Answer 1

Sin is a summoned creature, a magical super-weapon and 'armour' for its summoner, Yu Yevon. Yu Yevon summoned it into existence as his home city of Zanarkand was on the brink of destruction by forces from a rival nation of Bevelle. For a thousand years since, Sin has terrorised the world of Spira, stifling progress as it seeks to destroy habitations that grow too big or become too advanced.

Yu Yevon passed on the secret to temporarily subduing Sin. A summoner, with the willing sacrifice of one of his or her guardians, can summon a creature called a 'Final Aeon' with the power to defeat Sin. However Yu Yevon then possesses the Aeon and shapes it into a new Sin, though this grants Spira a period of 'calm' as it takes time to regenerate.