Exacids are the enforcers of The Dark Peak: biologically engineered crabs with armoured shells and heavy lateral pincers, created by The Nascenti and loyal to them with the uncomplicated fidelity of things built for a single purpose. They appear in bipedal form with some regularity, and the population of Hellsborough has learned to consider this unremarkable, which is a fact worth sitting with. Whether The Nascenti select for diminished cognitive capacity or whether the work produces it over time has not been established - either answer is unsatisfying. They will attempt the same approach three times before pausing, and that interval is your window. The problem was never the Exacid. The problem is what sent it, and why whatever sent it has decided not to show itself yet.