Ok ok theory time!!! The Will of the World has destroyed the world's defense mechanisms as the Great Spirit said. And the actual will and fate of the world isn't favorable towards humans. So this Will of the World, I think, is a God. A being above the world, probably someone who was formerly a human and ascended.
Great Spirit said that MC has a similar but opposing power. MC's nigh infinite miasma/mana might've turned him into something close to a God. Not quite there yet but he's definitely touched the boundaries.
If I'm right, or at least close to the actual truth, the final answer might be a battle between the human god and the demon god.
GREAT SPIRIT GOT THAT GYAAATTTT