This is about a former Mennonite town in Manitoba. the story part describes the slow death of the town as most of the people drift away, leaving behind only the households most invested in the lifestyle, land, and religion. At some point, they did something extremely awful in the name of the God they worship. this might have been not actually human sacrifice or execution but more along the lines of driving people to suicide through shunning or a similar practice (but you can also think of it as some more brutal crime, it's ambiguous). Naomi, the person first mentioned in the secret words, was opposed to the mistreatment but failed to prevent it.
whatever happened was so against the nature of the God whose name they did it in that it caused a very small injury to God Himself. That wound became infected by some otherworldly thing influenced by the twisted beliefs of the townspeople, and the infection had a very direct and awful impact on everyone who remained in the town. (this idea is heavily inspired by Imago, although this is a very small-scale thing compared to that of course.)
the infection has attributes related to both the God it infects and the religion that its 'followers' practiced. it cares about names and belief, and the best way to contain it is to deny and ignore it as much as possible, similarly to how Mennonites tend to shun and ignore wrongdoers.
Initially the Foundation wanted to ignore it forever, but it demonstrated it is capable of spontaneously spreading bloodless wings wide over the plains and that's bad news so they came up with a plan to stop it, probably decoded from information from the "song" in the area (which they banned everyone from ever listening to as soon as they got all the information they needed from it.)
to kill it, the Foundation is trying to read a series of accusations directed at former members of the town, which causes "fruits to wither", whatever that means. the infection can read the minds of people within the compromised area, so the accusation must be delivered by someone who doesn't know what it means and doesn't know the effect it will have when read.
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