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 MagiEdit
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. One example was the Mhachi tricked a Nymian sea galley into bringing an ampoule to their home nation containing a voidsent named Bitoso who unleash a devastating plague called the "Green Death" that transformed many Nymians into tonberries. But the mages of Amdapor developed White Magic to counteract the dark arts used by Mhach, sealing away the voidsent Diablos when he was sent to attack Amdapor.
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 FloodEdit
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 an airship powered by a thousand voidsent. However, even the mightiest voidmages could not completely contain its main power source, the voidsent Scathach, and soon the vessel became a derelict in the Sea of Clouds.
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 as a consequence of remnants from Sixth Umbral Era were unearthed by Bahamut's fury during the Seventh Umbral Calamity.