Let's do some theorycrafting.

Let's say "Hikaru" is Nonuki-sama, and it took Indo Hichi's (the wife's) head's place after "Hikaru"'s great^n grandfather came back from the mountain. This is probably what the author want us to believe, given that the head temporarily regained "life".

Then, as Yoshiki pointed out, it doesn't explain why the diseases / plague / famine stopped only after this incident. That means the impurities were already there in the village to begin with, before "Hikaru" / Nonuki-sama descended from the mountain. Clearly there's a contradiction of sorts.

The easiest explanation I can come up with to resolve this contradiction - probably obvious to many of you already - is that there is at least one other deity in the picture besides "Hikaru". This other deity may be the real Nonuki-sama, and "Hikaru" the deity was simply unrecognised. Or perhaps it's the other way round, but I don't think it matters - I am just going to refer to other one as Nonuki-sama for convenience.

"Hikaru" doesn't seem malicious, so I'm guessing it was probably Nonuki-sama, not "Hikaru" that caused the plagues in the village originally. After "Hikaru" descended from the mountain in the form of Indo Hichi's head, it may have encountered Nonuki-sama. They may then get into a fight, with the mass death of the villagers being collateral damage.

Whichever one was losing the fight fled to the mountain, with the other one in pursuit. There in the mountain, "Hikaru" and Nonuki-sama fought to a stalemate and kept each other in check, in a deadlock. With Nonuki-sama stuck on the mountain after the incident, the impurities disappeared from the village.

Back to the present, the balance has been disrupted. With "Hikaru" descending from the mountain, Nonuki-sama followed suit, bringing the impurities back to the village again.