Diamondheart Die-off

The Diamondheart Die-Off is a mysterious natural disaster that killed much of Diamondheart's population during the 1st century. To this day, no one knows exactly what happened, though most people believe the answers lie somewhere in the depths of Auras Pond.

What people know

While there is a lot unknown about the Die-off, historians have compiled some likely facts based on the few remaining texts from the time:

Auras was probably the capital of Diamondheart and that this was the center of the event.
The population of Auras appears to start decreasing several years before the event, indicating they had some warning of what would happen.
It occurred at the end of the 1st century and killed about 80% of the people living in the area, though there were settlements far enough from Auras to be mostly unaffected.
Those who did survive fled north to cities that were still standing.
The king of Diamondheart died in the disaster, but an appointed heir started a new capital further north.
The Thisaazhou and possibly other minority ethnic groups likely formed from the scattered groups that remained.
The die-off also caused a technological backslide.

What people believe

The leading theory as to the cause of the die-off is a massive earthquake. This theory states that the mysterious rings of Auras were built in an attempt to contain the quake, but the strength was too strong and broke several. If the rings were intended to direct the power to the center of the rings, it may have also been what created Auras Pond. It is also possible that the people of Auras built the binding rings to prevent a magical rift, like The Rift of 1920 but the technology backfired, making it much worse. A third theory states a pandemic wiped out the urban centers, causing less damage to the more rural north.

A variety of less logical rumors has also flourished in the midst of the mystery:

That the city was attacked by some sort of mythological monster. This stems from a 2nd century writer who claimed to have scryed into the past and seen monsters whose description resembles the Zenxonian myth of Janishbu.
That Auras was invaded by dragons. These claim teeters on the idea that the long-lived dragons in the area have refused to shed any light on the situation. But seeing as they were cast out of Auras several decades before the die-off, they likely don't know.
The city was invaded by an evil supernatural force. Which force this is, exactly, depends on a person's specific beliefs. Regardless, this rumor usually states that the evil now lies trapped at the bottom of the pond, and as such, no one should venture there.

CW: death, disaster, pandemic (mention), monsters