The Sundering was a cataclysmic event described in Final Fantasy XIV.
In the ancient past, there existed a world rich in aether, Etheirys, inhabited by a race that later came to be called the ancients. The ancients were the architects of life of the world, using their creation magicks to develop concepts of different kinds of animal and plant life. This society was ruled by the governing body the Convocation of Fourteen, a cabinet of fourteen ancients considered as peers within their areas of expertise.
Within a creation testing facility called Elpis, its main supervisor, Hermes, pursued a personal research into the meaning of life. To this end, he created the Meteia, beings sharing a collective consciousness from dynamis, an energy distinct from aether that relies on emotions. Since the space outside Etheirys is devoid of aether, Hermes needed a different energy source to send the Meteia to visit distant stars and learn their reasons for living. He kept one of the Meteia by his side as a receiver and a companion.
What Hermes did not anticipate was that the Meteia discovered mostly dead or dying worlds, unable to provide answers to his question. With every dead world the Meteia discovered, the despair they felt affected their collective consciousness, and they unknowingly projected their despair to worlds that still had life, resulting in those worlds destroying themselves, like the Omicrons of the planet Alphatron.
When the Meteia prepared to deliver their final report, Hermes was being scrutinized by Emet-Selch so that he would join the Convocation as "Fandaniel". During this he and his escort, Hythlodaeus, encountered an aetherically thin person, whom they assumed was a familiar of their friend, Azem. When they met the previous Azem, Venat, she detected this person carried a protective spell of her making, even though she had never met them before, realizing they hailed from the future. This person, known as the Warrior of Light, revealed to the three ancients that in their near future, a cataclysmic event that would become known as the Final Days would destroy Etheirys. Emet-Selch refused to believe the tale, but nevertheless investigated Hermes more closely. Venat and the Warrior did the same, and they came together to inform Hermes.
Just then, the Meteia delivered their report, which left Hermes distraught. He needed to hear all of it and spirited Meteion away to Ktisis Hyperboreia. The others gave chase and cornered him. The report concluded that life is needless suffering and despair, and it is in the best interest of everything living to succumb to inevitable death to be free from suffering. The Meteia intended to sing a song of oblivion to put the dynamis of the universe into motion and destroy Etheirys. Hermes allowed Meteion to escape, but Venat placed a tracking spell on her. Hermes activated Kairos, a memory alteration device to erase the memories of all those involved, declaring that mankind has to find the strength to overcome the apocalypse or die. Emet-Selch and Hythlodaeus ensured that Venat and the Warrior escaped, while theirs and Hermes's memories were erased.
Venat was now the only ancient aware of the Meteia, but she could not let this knowledge become public, as it would doom the world beforehand. She needed to carefully gather supporters, as she could not rely on the Convocation, with Hermes elected to the seat of Fandaniel. As the Final Days arrived, the Convocation, except for Azem, who vacated their position, came together to study the cause of the Final Days. They were never able to determine the source, only that the phenomenon caused all aether currents around Etheirys to stagnate. Hermes theorized that if they summoned a powerful being aligned with forces of activity and growth, it could agitate the currents and stop the Final Days. The Convocation sacrificed half of ancients' surviving population for the ritual to bring forth this being, the primal of Darkness, Zodiark. Zodiark's power generated a protective shroud around Etheirys that halted the Final Days.
Venat and her followers knew that summoning Zodiark was a temporary solution at best since even He could not solve the outlying cause. None of the Convocation agreed to find a permanent solution; instead, they intended to sacrifice even more for Zodiark so that He would restore the world ruined by the Final Days. Venat's followers disagreed as the price was becoming too high, and the Convocation's decision to cling to the past now lost would only repeat the ancients' plight. Venat decided that they must bring forth a primal of their own, that of divine Light, Hydaelyn, who would serve as Zodiark's jailer since Zodiark could not be destroyed without bringing back the Final Days.
To summon Hydaelyn, Venat's followers sacrificed themselves so that Venat can become Her heart, as she was the only one with the strength to stand against Zodiark. When this was done, Hydaelyn went into battle against Him. Her only recourse to defeat Zodiark was to rend Him, and the entire world alongside him, asunder. This final blow caused the Sundering, and all of Etheirys and its inhabitants, body and soul, were sundered into fourteen shards. The world split into the Source and thirteen reflections, numbered from First to Thirteenth. The sundered people of these fourteen worlds had only a fraction of the aether from the ancients, and they also forgot their past lives as ancients. Hydaelyn held onto hope that these fragmented, imperfect and mortal beings would be capable of living in happiness and hope despite the suffering they were forced to endure, and the inevitable death they would eventually succumb to.
An intentional flaw in the Sundering left Emet-Selch's soul intact, as well as the souls of two other members of the now defunct Convocation, Lahabrea and Elidibus, who were nearby. These three unsundered souls established a new order, the Ascians, with a goal to find their sundered brethren and restore Zodiark and the world they loved by rejoining the reflections back into the Source through series of Umbral Calamities.