What I am about to tell you is the unclouded truth.
Some of the backstory to Final Fantasy X is told in the Final Fantasy X Ultimania Omega official timeline. Twenty-eight years before his death, Jyscal and a human woman had a son, Seymour, who was viewed as an abomination by both races. Three years later Jyscal became the leader of the Guado and spread the teachings of Yevon to his race. Five years after this, when Seymour was eight years old, to soothe the waves of a possible uprising, Jyscal had Seymour and his mother sent to the palace at the remote island of Baaj where Seymour was trained in the ways of a summoner.
When Seymour was ten years old he and his mother embarked to Zanarkand to defeat Sin. Seymour's mother knew she would soon die and feared her son would have no place to belong to in Spira, hoping that giving him the chance to defeat Sin would win Seymour acceptance. Seymour's mother knew of the mechanics of the Final Summoning through Jyscal, as the Guado possess their own records that tell of Spira's sealed histories in fragments. Jyscal was aware of Seymour and his mother going to the Zanarkand ruins, and privately supported it. When his mother sacrificed herself to become his Final Aeon, Seymour was horrified and broke off the pilgrimage and returned to Baaj alone. The aeon Seymour's mother had become, Anima, and the knowledge of his mother's sacrifice, gave Seymour terrible ideas about life and death, and he began to form a messianic complex. With a changed outlook, he created a plan to "save" Spira from the cycle of death by becoming Sin himself.
I would think that he was sent once...but he stayed on Spira. Something, a powerful emotion, could have bound him to this world. Such things happen."
"That's against the rules, isn't it?"
"It means he died an unclean death.
During High Summoner Braska's Calm, the Guado's internal strife subsided, and Seymour's exile was revoked. An 18-year-old Seymour returned to Guadosalam to assist Jyscal in the duties of a priest. A year later, Jyscal, leader of the Guado, and Kelk, leader of the Ronso, become Maesters of Yevon as part of Grand Maester Yo Mika's "sub-races appeasement policy."
Ten years after returning to his home village, Seymour killed his father to succeed him as Maester. Lord Jyscal's soul was sent to the Farplane, but most unusually, he returns for a few moments to the world of the living to give Yuna a sphere documenting Seymour's evil motives. Jyscal is sent once more by Yuna, but Jyscal's sphere reveals to the party Seymour's true nature. After watching it, Yuna resolves to marry Seymour and convince him to turn himself in for Jyscal's murder.
The plan fails as Seymour attacks Yuna and her guardians and is killed. He stays in Spira as an unsent, but when Yuna and her guardians are put on trial for the murder of a Maester and she reveals Seymour's crimes to the Grand Maester, they are brushed aside, exposing the true corruption of Yevon's Maesters. Tromell laments that the Guado are doomed without a leader following Seymour's death.
Two years after Sin's defeat, Pacce and his friends locate a mysterious dungeon under Bevelle. Yuna, Rikku, and Paine traverse this labyrinth, the Via Infinito, and discover Jyscal's soul trapped there (despite previously having appeared in the Farplane), along with several others, such as those of Maesters Kinoc and Mika, Yunalesca, and Lord Zaon. Jyscal transforms into the Black Elemental to fight the party.