Flood of Light
The Flood of Light was an event which occurred approximately one century before the events of Shadowbringers, on the Shard of Hydaelyn called the First, the shard closest to Hydaelyn's Light. The Ascians intended for this deluge of light aether to trigger the Eighth Umbral Calamity on the Source, the Calamity of Light, consuming the aether and soul-shards of the First and hastening the Rejoining of Zodiark. The Flood erased all life, and all elementally-aspected materials like the waters of the planet's oceans, leaving only an empty, white-sand wasteland with greyish, crumbling rocks. Only Norvrandt was spared.
The Ascians spent many years, from the perspective of the First, preparing that world for its fate. They purged or concentrated sources of darkness and focused the sentiments of its population on stasis, passivity, and resignation. Two Ascians, Loghrif and Mitron, were both slain by the First's Heroes of Light, led by Ardbert, along with the Shadowkeeper, the greatest single source of Darkness in the shard. These events combined to overload the First's aetheric balance with light, which would have consumed the entire planet and triggered a Calamity on the Source. However, Minfilia, acting the Voice of the Mother, used her newfound powers to halt its progress just as it was beginning to destroy Norvrandt. The destructive potential of the Flood can be seen in Amh Araeng, where it almost completely obliterated the city-state of Nabaath Araeng before Minfilia appeared.
Strangely, though it is called the Flood of Light and clearly produces bright, white, glowing effects, it was in fact caused by an overabundance of umbral-aspected aether, as sages of the Source would call it. That is, aether with a "negative" aspect that subdues and stills the flow of aetherial currents, which is traditionally associated with Zodiark. On the First, however, the terms are "Darkness" and "Light" instead of "astral" and "umbral," respectively. Urianger notes these differences and seems unsure which system is more accurate.