Journey's End

The Bond

As the last survivor of Zanarkand with the knowledge of how to create Fayths, Yunalesca is the only person who can grant a Summoner a Final Aeon. The ceremony is a mystery to everyone who has yet to visit her in the Yevon=Dome. All that is known is that the Summoner dies after performing their Final Summoning against Sin and that a bond is necessary.
"There must be a bond, between chosen and summoner, for that is what the Final Summoning embodies: the bond between husband and wife, mother and child, or between friends. If that bond is strong enough, its light will conquer Sin."
          –Yunalesca at Yevon=Dome
Yunalesca turns the chosen candidate into a Fayth which the Summoner then calls to fight Sin. The stronger the emotions and the bond, the stronger the Aeon. After defeating Sin, the Summoner dies as Sin breaks the connection between Summoner and Aeon.

A Summoner can also choose not to use their Final Aeon on Sin. One of them was Seymour, who seeked more immortality and power. As someone who knew the truth about the Final Aeon and Sin, he wanted to become the Final Aeon for someone who would definitely defeat Sin. By doing so, he could merge with Sin and control it to destroy Spira.

Truly Defeating Sin

A thousand years ago, Yunalesca turned Zaon into the Fayth for her Final Summoning. With the love they shared and the strong connection with one another, Zaon was powerful enough to defeat Sin. During the time following Sin's defeat, it took the Final Aeon which had defeated it and began creating a new armor. Since Sin was halted from destroying Spira, this became known as the Calm.

When Sin returned, many Summoners died trying to do what Yunalesca did. Yet each time Yevon=Ju transferred to and possessed the Final Aeons that battled it. Sin would then be reborn, placing Spira in an endless spiral of death. Only if Yevon=Ju had no more Aeons to possess could Sin be truly defeated.

Truly defeating Sin means piercing its armor, defeating the immortal conscience of Yevon=Ju, and then having a Farplane Sendoff performed by a Summoner. However, Final Aeons were the only known method of piercing Sin's armor...
Song of Prayer Princess Summoning End