Thalassic Annihilation

Humans evolved in northern Hatag, spreading to the main continents when sea levels were much lower as result of an ice age. At the time, seafaring technology was enough to traverse the small and medium distances between land as well as to island hop. The technology allowed them to make the short leap to Aowa and the large leap to Doricia (or whatever it's called), as well as to a volcanic island chain in the Haxamic, now known as the Milynv Islands. Humans who got trapped there eventually evolved and became their own human subspecies.

And then something very strange happened. According to historians, anthroposophers and archeographers, it seems there was an ancient event which suddenly and mysteriously set back seafaring technology and made people flee back from the coast for at least a millenium. This event is evident across all of the coastlines of the world from terrographical data and all human accounts are eerily similar, describing boiling bloody seas and human screams on the wind, amongst other ghastly occurrences I can't think of yet. It caused quite the spark in popular culture when the evidence was brought to light, and had an effect similar to that of the discovery of dinosaurs on the minds of the people. It is known amongst scholars and philosophers as the Thalassic Annihilation but to everybody as Screaming Seas.

Occurred around 10000 BCE and may or may not be related to the advent of Hylla. The sea seemed to froth and boil without wind as far as the eye could see. 10m high waves, red/grey skies, freakish hurricane winds. And the human records all went silent for a couple thousand years and they emerged with seemingly no knowledge of seafaring and boat construction. Several institutions have already been set up to study and uncover what must be the biggest mystery in human history (including the Cult of Hylla), the most notable being the League of the Hare at Crissindrey University, which has a huge library full of reports, theories, speculations and tomes about the Screaming Seas event.

Screaming seas mysteriously coincides with the coming of Hylla. Theory of the Cult, and suggests existence of God. Circa 10000 BCE