"The 8th Umbral Calamity spelled an end not only to the civilizations of Eorzea, but also the Garlean Empire. Survivors included Cid Garlond, who continued operating Garlond Ironworks. Inspired to undo the Eighth Ardor, he used the concepts of traversing the rift observed in his encounters with Omega and the time travel theory observed in his encounters with Alexander to work out a way to traverse time and space to reach the First before its Rejoining. The heroism of the Warrior of Light, who perished in the Calamity, inspired many people to lend support to the project, as stories of the legendary hero kept hope alive in these dark days.
The research passed to a descendant of Biggs, who developed the technology required to awaken G'raha Tia within the Crystal Tower two hundred years after the Calamity, as they hoped to use the tower as a vessel. They would travel to a time after the hero's victory in the Dragonsong War, and G'raha, having control over the Crystal Tower, would travel within it to the First.
Because time flows differently between the Source and its shards, G'raha Tia arrived one-hundred years prior to the desired time, but survived under the identity of the Crystal Exarch, protecting the refugees of the devastated First in a city formed around the Crystal Tower, and eventually called the Warrior of Light to the First. The Warrior of Light, under the mantle of "Warrior of Darkness", returned balance to Norvrandt, removing the calamitous potency of Black Rose on the Source (the supply of which was separately destroyed) before it could be employed and preventing the First from being used for the Rejoining, thus averting the Eighth Umbral Calamity altogether. The alternate timeline following the Eighth Umbral Calamity was not erased from existence, but began to rebuild civilization under Garlond Ironworks' descendants and Midgardsormr." -FF wiki