Dreams.

It turned out that another tribe inhabited the island (due to the tide, it was now more of an archipelago), and rumors said they possessed dark magic. They lived in isolation, built a village with straw houses, and surrounded it with a high palisade. It was decided to send one of the natives, the youngest and bravest, on a reconnaissance mission. He managed to reach the palisade of the savages by boat, but then perished, presumably due to magic.