Yu Yevon

Yu Yevon was once the ruler of Zanarkand during the war with the Machina city of Bevelle. Seeing how his people were faced with imposible odds and doomed to perish he wished to preserve the memory of his home after it would be destroyed so he told all his people to go to Mt.Gagazet and turned them all into fayth for his summouning of Dream Zanarkand a city he placed far out in the ocean away from the mainland of Spira so it would not be desturbed. He also created Sin out of pyreflies as means to pretect himself while he made his perpetual summoun, this process over hundreds of years eventually corrupted Yu Yevon and his purpose of continuing the Summouning at all costs degraded his humanity. Sin was set on a sort of automation where it would destroy all settlements with anything approaching the technology to find Dream Zanarkand this is why a thousand years later the world of Spira has regressed to a much more rural less developed society where machina are outlawed because of the beleif that they caused Sin's existence. However a method of destroying Sin was created but the being used to do this: the final summouning would be used to create a new Sin which would merge with Yu Yevon trapping the fayth inside until this new Sin was destroyed. Sin attacked Dream Zanarkand transporting Tidus to the Spiran mainland and setting up the eventual downfall of Sin and Yu Yevon and bringing about the Eternal Calm. Yu Yevon is also the father of Lady Yunalesca who grants summouner the means to defeat Sin and gives the party much of it's information on Yu Yevon while explaining his importance to Spira and explaining why she believes he must exist eternally for the cycle of death to continue and for Sin to continue.