From Crunchyroll:
In the Edo period, there was a shrine maiden called "Itsuki-hime" in the mountain village of Kadono.
Jinta, a young man who acts as Itsuki-hime's guardian despite being a stranger, encounters a mysterious demon who speaks of the far future in the forest where he went to defeat it.
From Edo to the Heisei era, this is the first volume of a huge Japanese fantasy series about a demon man who travels through time while continuously questioning the meaning of wielding a sword.