You were actually spot on with most of these! This SCP is the second entry in my Pennsylvania Dark series, about a secretive cult in rural Pennsylvania called the Nachtkarich — but they're not a doomsday cult. They have access to some rudimentary occult techniques, mostly around long-distance telepathy and information transfer, but their primary weapons are lots and lots of guns and home-made pipe bombs. They're fanatics, ready to fight to the deaths of every man, woman, and child in their village against the evils of the Foundation/United States government/any authority figure whom their leader tells them to hate.
They're aware of the Foundation, and allowed the Instruction Manual (of which this is, indeed, the only copy) to fall into the Foundation's hands. They're using it as a kind of psychological warfare, sending the Foundation on a wild goose chase after fictional copies of an instruction manual for a fictional dark god.
The kill count is made up, as well, and is an attempt to make the Foundation fearful and uncertain. The only thing that was real is that they hacked a Foundation computer terminal, and killed a Foundation employee in a drive-by shooting.
I'm curious to know what plot thread you were imagining?
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