There were new gods, and before them old gods. Before them, there were elder gods, and before them there were the ancient gods, and so on through the ages, back to when the world was young and fresh, unknown and frightening, in the eyes of a new species of primate. They spread north and west and east, further and further, beyond lands of ancestors swept aside in the new tide into ones entirely new, strange and wonderful, lands viewed by these new beings capable of experiencing and comprehending novelty and awe. In those ages the First Gods were brought into being, nameless spirits of land and sky and nature and inscrutable fate. These were not the weaker, limited, constrained, anthromorphized gods of far future times, constricted by their names and roles and turned into humans made large, sitting on clouds, their powers sapped and used for mundane tasks. They simply were, and were in.
When some the first seafarers (who gave themselves names, but did not give names to their groups - call them the People), traveling on bits of wood across the endless waters, reached an isolated but verdant island (which they did not name either, but simply thought of us the Land until they became so used to them that it simply was), there became a god who was and was in the island and fish-filled seas around. A tropical paradise, all that the small group of People and their descendants would need in perpetuity, where each day and lifetime could blend into the next, and time could become blurred and confused and endless. And so the god could see past, present, and future, mixed together. When life was often inexplicable and without understanding, so too were the gods, back in those times. In the far, far future were dangers and terrors beyond imagining. But what was the future when all time was the same? There were dangers beyond the Land, dangers to the People. The only recourse was to hide, to stay, to fight. The First Gods did not give commandments, could not give orders, but this one, in exchange that it might exist for tens of thousands of years, long after all the other First Gods would fake and perish and melt away into newer gods - this one could give a warning.
The Beyond, the Outside, was bad. It was death and fear and plague and evil. Fight it, and drive it off, forever.
So they did.
Legends grew about the island of people who killed anyone who came too near, without exception. Myths spread about the savage cannibals, though the People did not eat those they slayed and simply buried the bodies on the shore, to keep the disease of Beyond at bay. For a short time, the Land was called North Sentinel Island, and the People the Sentinelese by ones Beyond. The People were not swept away by future waves of people, nor destroyed by colonization as were their long-separated relatives. They fought fiercely, keeping the world at bay, not flinching in their duty even as the attempted invasions became more and more strange, as shining birds of hardened stone flew over and new strange lights began to move quickly among the stars.
When the Rash came, and swept away so many of the other peoples as they had once swept across the land and swept away prior peoples, the old power before magic was still on the Land. A few, very few fools from Beyond attempted to crash on the Land to escape the Rash, unknowingly bringing it with them, but they were shot by arrows at a distance and the Land was kept safe. Strange new monsters began to roam the waters beyond the reefs of the Land, but the old power of protection, forged from tens of thousands of years of defense, keep most of them away. For those few monsters that stray too close - the People reserve their precious steel-tipped arrows and spears, made from bits of hacked-off boats, for those.