Zaon is a non-playable character in Final Fantasy X, who also appears briefly in Final Fantasy X-2. He was the husband of the first high summoner, Lady Yunalesca, and they both lived in Zanarkand over 1000 years before the events of Final Fantasy X.
When Yu Yevon first created Sin and went mad, Zaon became Yunalesca's guardian. She sacrificed Zaon as a vessel to create the first Final Aeon, and herself for the power to make him strong enough to defeat Sin, all for the brief respite from Sin's depredations that was to be known as the Calm.
Yunalesca remained in Spira as an unsent, making the Zanarkand Dome her abode where she greets summoners who complete their pilgrimage to grant them the Final Aeon by converting one of the summoner's guardians into its fayth. The Final Aeons were not only tasked with killing Sin but doomed to be possessed by Yu Yevon's spirit and become the new Sin. Thus, Zaon was Sin's first slayer, Yu Yevon's first host, and the second Sin. After being defeated as Sin, Zaon's unsent spirit found its way to Via Infinito beneath Bevelle.
A thousand years after Zaon first became Yunalesca's Final Aeon, many summoners have attempted to complete the pilgrimage to bring about the Calm. Seymour Guado welcomes Yuna and her guardians to Guadosalam and shows them visions from his sphere, including of Yunalesca and Zaon. As Yuna's namesake, Seymour reminds her Yunalesca did not defeat Sin alone. "To defeat the undefeatable Sin, it took an unbreakable bond of love, of the kind that binds two hearts for eternity". This is his preamble for a proposal of marriage to Yuna.
As Yuna's party arrives to Zanarkand they find Zaon's fayth in the Chamber of the Fayth, but it is just a mere stone tablet, its spirit lost.
Yuna, Rikku, and Paine can enter Via Infinito and witness what appear to be Yunalesca and Zaon embracing, as they had in Seymour's sphere, before Zaon disappears and Yunalesca is replaced by her fiend form, Chac. Later, they may encounter Zaon in a fiend form called Paragon. The girls beat him, but he is destroyed by Trema.
Zaon is fought as a fiend similar in appearance to the Omega Weapon. He boasts high HP, Strength, Defense, and Agility, yet his Magic and Magic Defense are average. His normal state gives him access to Attack, Big Bang, Genesis, and Supernova, however when under the effects of the oversoul, Paragon gains access to tier three Black Magic spells, Ultima, Holy, Demi, Dispel, and Judgment making him a more fearsome foe. Oversouling Paragon is required to complete Shinra's Bestiary.