The Great Vanishing

The Great Vanishing is an event that took place in the Thenian archipelago state of Trece Isla sometime between the year 1000 and the 3300s. When it occurred is unknown, but archaeologists have concluded it was most likely in the 2100s.
When it happened is obviously a subject of debate, but so is exactly what happened. All that's known for sure is that a cataclysmic event caused the people that lived in the area of the Great Temple of Coscarra (and even surrounding islands) to hastily abandon their settlements. The state of Trece Isla was not yet officially founded, but thousands lived there, and then very quickly, they simply disappeared.

Theories

There are two main theories that surround the Great Vanishing, although there are perhaps as many theories as there are people in Thenia.
The first main theory is that the people were attacked, either by people outside of Thenia, or other Thenians (although it was not yet Thenia). It's known that the Great Temple of Coscarra was home to the Cult of the Sacrificed, and it's possible that outsiders heard of the human/animal sacrifices and decided to attack. This then led to the people of the island to disperse to other parts of the island (or they were killed).
The second main theory is more supernatural: it's said that the victims of the human sacrifice came back to the Great Temple to exact revenge. The people of the future Trece Isla were so terrified that they then fled.
Other theories postulate that a pandemic hit, there was in-fighting that led to their own destruction, or some sort of famine.
Frankly, any theory is plausible, as it's widely recognized in Thenia that the dead are able to visit the living, no matter the religion.

Future

The people of Trece Isla hope that archaeology will uncover more answers, as it's an intriguing mystery to both Islans and other Thenians as well. Archaeology of the Great Temple of Coscarra and surrounding areas continue.