The oni-wives migrate on schedules unknown to mortal scholars.
"Migration" probably isn't even the right word, as it implies something regular, instinctual, inevitable, and purposeful. But the coming of the oni-wives is none of those things, save perhaps "inevitable."
The migration will happen, and the oni-wives will come. Will you be ready when they do?
They emerge from their mountain caves and redoubts and fastnesses, gripped by wanderlust. And where they wander, things change.
The changes are small at first. One of the witch-ogres arrives in a village and saves a child. Another finds a wolf whose leg is stuck in a trap and she sets it free.
But soon they grow large. An oni-wife destroys a slaving ring. An oni-wife faces down an army. An oni-wife destroys a city.
And before anyone can fully adapt, they disappear, returning to their mountain holds (or sometimes slain by angry mobs) and waiting for the next cycle.
Why do they do this? The few scholars who know of them have many theories but few answers. Some say they are emissaries of the numinous, go-betweens and messengers among the gods, the primordials, the folk of the world, and the forces of magic. Others say they are entropy incarnate, yet another mechanism by which civilization is doomed to fall. Still others claim they are the immune system of a healthy cosmos, a process by which the world keeps its systems vital and keeps its ages turning.
(Sages also wonder at their names. Are they truly kin to ogres or oni, or are these just poetic epithets? What do they do when they are not in the world? What drives them to come and bring change with them? Do they hear the songs of the gods and the music of the spheres keening constantly in their minds?)
It ultimately does not matter from whence they come or why they come or even when they come. What matters is that they will come, and you (or your children, or your children's children) will have to adapt to the change they bring with them.
When they come, will you be ready to greet the world they bring with them?
