Sixth Umbral Calamity

The Sixth Umbral Calamity was a cataclysmic event in the backstory of Final Fantasy XIV. Known as the Great Flood, it happened 1,500 years ago in Eorzea during the height of the War of the Magi, and signaled the end of the Fifth Astral Era.

The War of the Magi
During the Fifth Astral Era, Eorzea was dominated by three major powers: the scholars of Nym in Vylbrand, the white mages of Amdapor in Twelveswood, and the void mages of Mhach in Yafaem. For unclear reasons, a war broke out, with Mhach utilizing increasingly destructive applications of voidsent to combat their foes. While Nym was cursed by a mysterious plague, the mages of Amdapor developed White Magic to counteract the dark arts used by Mhach.

This wanton abuse of the lands' aether provoked the anger of the Elementals. From their collective wrath came Oha-Sok, an unaspected elemental that decided to to eliminate the warring factions with a catastrophic flooding of the realm.

The Great Flood
The extent of the destruction is unknown, but all three cities of magic ceased to be as the waters rised. According to legend, it is said the Zodiac Braves appeared with legendary weapons to guide Eorzea through the Sixth Umbral Era and wage war against demons known as "Zodiacs."

The Mhachi attempted to ride out the rising waters by building the Void Ark to take flight. However, even the mightiest voidmages could not completely contain its main power source, Scathach, and soon the vessel became a derelict in the Sea of Clouds.

Aftermath
The recede of the flood waters led to the Sixth Astral Era, and in time rise of the modern city-states of Ishgard, Limsa Lominsa, Ul'dah, and Gridania. The Elementals maintained a tight grip over the Twelveswood, who only permitted Gridania's existence through their mediators, the Seedseers.

In the fifteen centuries of the Sixth Astral Era, the great flood became a distant memory. Its legacy was only truly rediscovered thanks to the upheaval wrought by the Seventh Umbral Era, when many secrets from the time of the Sixth Umbral Era were unearthed by Bahamut's fury.