1000 years ago. It was the Machina War between Zanarkand and Bevelle. Despite being a great machina city, Zanarkand relied on summoners to fight. Bevelle's machina though, assured their victory and Zanarkand was doomed to destruction. Yu Yevon, not wanting Zanarkand to die forever, decided to preserve its memory. He called on all the surviving people, turned them into fayth and placed them on Mt. Gagazet, protected by a dense mist.
From their memories he created Dream Zanarkand and set it far out at sea. To protect himself and his city while he summoned it, Yu Yevon amassed a great amount of pyreflies to form an armor around him; an armor called Sin. Thus, Sin was born, with its purpose to protect the Dream Zanarkand by any means possible. Summoning both the city and Sin was a greater strain than Yu Yevon could handle; thus only his instincts to protect his beloved city remained and the spiral of death started. Jecht and Tidus, father and son and citizens of the dream city, became real when they touched Sin and were brought to Spira to end the spiral of death that has ravaged it for 1000 years.