If anyone feels that this feels like it came out of nowhere...
I think it is because we never saw the "true prologue" of the story.
For example, where did the sword come from?
Why does he have a presumably Japanese human soul inside him?
Why doesn't he remember anything from his past.
Is that power something he gained from the gods to begin with, or gained, or somehow lost?
It does feel like they pulled this out of nowhere since we have no clue what he is supposed to have in the first place.