Just my SPECULATION. I have no connection to the author and have no way to know what will happen.

I think the Nounuki-sama Hikaru finally understand the importance of lives. He will sacrifice himself and spread over the mountain, swallowing all impurities, present and future.

He does it out of love for Yoshiki and their friends. Yoshiki realises he loves the Nounuki-sama Hikaru, no longer the old Hikaru.

Nounuki-sama will become the guardian deity for Yoshiki, their friends and all the villagers. He will lose self awareness.

Total speculation. Feel free to share your thought.