The Starfall

The Starfall is a poorly understood event that happened roughly 1000 years prior to the current date of the world, but the effects are still visible. The Starfall was centered on Dujuku, a highly advanced civilization that surpassed any other nation that has existed since, and managed to single-handedly wipe it off of the face of the earth. All records of how it started have been lost, but it is theorized to have been the result of an experiment gone wrong, in which an unstoppable force of nature was released. The original event is often referred to as The Starfall, but the force of nature it released is referred to as The Starfall.
The Starfall itself is effectively a large storm cloud, roughly 10 miles in diameter, made of cobalt blue mist the still drifts over the land that was once Old Dujuku. The cloud at the center is not stationary and has a tendency to roam all around the Dujukan valley, but has never been recorded as going beyond the mountain ranges that encircle the region. The cloud structure at the center of the Starfall is the center for vortexes of very strong winds that are capable of damaging structures and makes it the most dangerous part of the region. However, the blue mist that drifts down from the Starfall is highly toxic to living things and has made all of the land within the Dujukan valley uninhabitable, even while the cloud is not present. The mist has a way of infecting and mutating living things that it does not kill in ways that make them vectors to spread the mist.
The Starfall is perhaps the singular most influential event in recorded history. It wiped out the most powerful nation in existence along with most of the scientific knowledge they had accrued. There are still relics of Dujukan technology spread throughout the world, but much of the knowledge about how they work has been lost. It also had disastrous ecological effects on the world, even outside of its immediate vicinity. The most notable of these are the spread of tainted creatures that were mutated by the mists of the Starfall, such as Purge Wraiths, that seem compelled to spread the fallout of the Starfall. The power vacuum created by Dujuku's absence sparked large scale conflict that, in combination with the those ecological effects, effectively destroyed most other nations in the immediate vicinity of Old Dujuku, further contributing to the loss of knowledge about the era.
One final effect of the Starfall was the ensuing diaspora of Dujukan refugees. Those that survived The Starfall were left homeless and sought to find new homelands. This resulted in a few isolated enclaves of Dujukan refugees settling across the continent. Most of these settlements failed or were absorbed into the nations where they settled, but one notable exception is the current nation of Shindus, which was at one point a new regional superpower but went through a significant dark age following the Fall from Grace.