The Darkness will be a creeping, otherworldly corruption that eats and spreads, spawning foul abominations with the strength of a dozen common men and will wipe out everyone of the Resistance and Magistrate in the future of the Realm.
It would be accidentally unleashed by the Magistrate who saw a chance at victory, to destroy the Resistance. For this they searched in the Shattered Desert for the shards of the moon, the stones that fell here, that contain a power they didn't really understand.
The Darkness is not of the moon. But somehow, it is harbored in the shards. After Jenos ascended into the stars, Jenos was told by the moon of the darkness.