Hmm I must get myself a copy of this book.Well, I'd have to assume it blows its own cover to the point where the idea that there's something in the lake that can create undead puppets and it wants to take over the world has became common knowledge. This still leaves it fairly well positioned, assuming it has enough puppets to fortify around the lake against attempts at invasion by unassimilated humans. The puppets have the advantage of being neigh invulnerable against eighteenth-century weaponry, given that they run on necromorph rules* and have enough intelligence to use tactics and weapons, but are heavily limited in number if humanity can surround the Tarn's territory, leaving it with only a finite number of victims to assimilate, then wear said limited forces down with superior numbers and logistics**.
* They can't actually be "killed", merely dismembered to stop them moving. Matchlock bullets are completely useless unless they happen to break limbs, blades ought to be more effective.
** The Tarn has a deteriorating gothic mansion and village. Minimal manufacturing capacity for firepower.