Eighth Umbral Calamity
|Spoiler Warning : This section contains spoilers for Shadowbringers!|
Within the alternate timeline leading to the Eighth Umbral Calamity, the Scions of the Seventh Dawn remained unaware of The First's existence, and Emet-Selch was able to trigger a Rejoining without opposition. The First in this timeline was completely consumed in a flood of destructive light-aspected aether.
Roughly 100 years later, on The Source, the Garlean Empire unleashed a deadly chemical weapon known as Black Rose against the Ala Mhigan Resistance,to devastating effect.
Summary of Events
While each of the inciting events described above caused a considerable loss of life, the Light-aspected nature of the First's rejoining is implied to have caused the effects of Black Rose to be far more potent than the Garleans had ever intended. 
In addition to the immediate deaths caused on it's release, Black Rose wreaked havoc on global ecosystems, making the land unsupportive, if not hostile, towards all kinds of organic life. The compounding death tolls from these after-effects led to the collapse of all known civilizations of the time, simply due to the small number of people who survived.
- See also: Shadowbringers
- [A World Forsaken] "The reason the sender had been unable to communicate with said comrades was simple: they had been annihilated by Black Rose, a poisonous gas developed by the Garleans."
- Light is associated with passivity, or dormancy, as revealed in Shadowbringers. Black Rose effectively induces a state of full dormancy on a living being's aether, according to the description of its effects in [A World Forsaken]:
"This ingenious weapon halts the flow of aether within organic beings, causing near immediate cessation of respiratory functions. Being highly potent and capable of rapid dispersal across a wide area, the majority of settlements in nearby Ala Mhigo were almost entirely depopulated in the aftermath."
- [A World Forsaken] "Contemporary reports state that the damage extended far beyond Alliance-held regions such as the Black Shroud and Thanalan to several imperial territories, resulting in significant loss of life."
- [A World Forsaken] "Black Rose wrought immeasurable destruction to organic life-forms, resulting in widespread environmental upheaval. The ambient aether ceased to flow along its normal channels and stagnated, rendering the land barren and uninhabitable. Inevitably, the wild imbalance of natural energies affected neighboring regions and, in some cases, led to sudden and unpredictable alterations in flora and fauna. Overnight, the crops on which man relied for sustenance became toxic, further increasing the death toll. In the aftermath of these unprecedented disasters, the communities formed by sentient beings fell into disarray, and all forms of societal organization, from small settlements to entire nations, were abandoned as individuals struggled desperately to avoid annihilation."