Looking for a manga with a tragic ending

Been seeking it for quite a while but i couldnt do it on my own, the premise is this:

• The story is set within a rural area where a boy meets a mountain spirit who manifests as a girl.
• Its an age progression story where the boy grows up and was conscripted into an army, the boy is shown growing older whilst the mountain spirit looks the same.
• They both have sex before presumably the last time they meet alive.
• The boy was hinted to be in a high position in the army and had a will which brought him back to the mountain as an urn.

Pretty much what i've been searching for but cant find, do let me know if anyone found it.