The scant records that exist, suggest that three thousand years after the first civilisations began on Nataya (as far back as any recorded history stretches), there was a global geological event. Continents shifted and separated. Volcanos erupted, creating new islands and wiping out some existing civilisations. This created a period of chaos for many years, as people had to relocate, and some cities were entirely wiped out. Some rifts between planes opened, which both added to the chaos as new beings and beasts came through, and also allowed for exploration to these new planes.
Divine Ascension
Though records from the time are few, and often thought to be unreliable, it's believed that most of the major deities that currently exist during The Second Calm, initially had significantly less power during The Era of Tranquility, but something occured during the First Great Turbulence that caused their ascension to the level of major divinity that they occupy today.