A murder, a massacre, a death camp.
How do you cleanse a place where something truly infamous has happened?
Not hard. You let the place of death become a place of life. You return it to the Mother.
Such a place becomes accursed. Only Earth herself can cleanse it.
Best to do what the ancestors did.
You raise a standing stone to mark the place. In due time, her own time, Earth will make it clean again.
Meantime, such a place is best avoided. 'Ware contagion.
How, then, do you know when the curse has run its course? Not hard.
When no one any longer remembers why the stone was raised, the place is clean.