Watatsumi

Long ago there was the one continent, and it was the mainland. The people living there were primitive, living in small groups rather than refined towns. The gods were only young, and didn’t have much influence on the people.
There was one creature everyone feared. It had come from the sea and consumed everything in its path, its winding coils would crushing all below it. All it knew was hunger, and no one was safe from its gluttony.
This creature did not hold grudges. It was not good, nor bad, it simply lived on its instinct alone. No one could hold this animal accountable, and there was no reasoning with a hungry beast.

The mainland wasn’t enough for this beast, and each night it looked into the sky, and it would see the moon glowing above. 
The beast was angry that it could not reach it. It longed to feel it disintegrate beneath its terrible teeth. The beast launched itself into the sky with its wide jaws reaching out to engulf the single light. The land was cast in shadow as it lunged, clamping down on the moon. But the beast was too heavy and the rock it had started to latch onto began to chip, and its grip loosened. It couldn’t hold on any longer, and as it fell from the sky, a chunk fell with it.
The beast fell into the ocean, exhausted. As it sunk to the ocean floor, its eyes gradually closed, until it was one with the seabed, fallen into a deep sleep.

Aside from the chunk that it had consumed, another, smaller, fragment fell with it, crashing not far from where the beast lay. It became [place].
The gods took over not long after. Certain the beast was gone, they guided the people to the new land, where they formed civilisation. A large number of people decided to stay behind at the mainland.
The gods grew their reputation as the years went by, either as peace keepers, or symbols of war. People began to forget about the being that almost had driven humanity from existence. The gods began to forget of their youth, and they became lustful for power. 

It was only the worst and most vengeful of the gods that remembered the devastating evil
 that had nearly destroyed them all.