Onigajo is Kumano's most spectacular coastal sight: a line of caves, ledges, and wave-cut rock walls facing the Pacific. The name means "demon castle", and the place does have a rough, theatrical quality, especially when the sea is moving.
A path lets you walk through part of the formation, with cliffs on one side and open water on the other. It is a strong first stop in Kumano because it makes the coast feel ancient, violent, and very close.