Omega was a monk of Yevon 700 years prior to the events of Final Fantasy X. Although the Yevon temples' teachings had become widespread, 600 years passed with no one able to defeat Sin. During that time, many doubted Yevon's teachings and thought about trying to defeat Sin in their own way; Omega was one of those who questioned the teachings.
Originally a priest of Yevon, Omega was executed for doubting the teachings and openly rebelling against the temples. Unable to go to the Farplane, his spirit lingered in Spira and he became a fearsome fiend. Omega's hatred for Yevon grew so great and intense that he spawned the lesser beast, known as "Ultima Weapon".
The islands where the fiend makes its abode is the Omega Ruins.
Omega (capital: Ω, lowercase: ω; Greek Ωμέγα) is the twenty-fourth and last letter of the Greek alphabet. It is synonymous with a set's end or limit.