A Creeping Tide (2019)

From publisher blurb:

"Saxford you say? Yea I know it. Small outpost on the far side o’ Lake Argos. Sure you want to go there, stranger? Lot o’ death in Saxford, I hear. Nay, not by beastmen nor barbarian… Saxfords’ are taking their own lives. Why? Har! Might as well ask why the sun sets at dusk, eh? Wodon knows, wanderer. Wodon knows."

In this adventure, Saxford’s outlanders (exposed to the Black Spire’s subtle taint for years) engage in increasingly bizarre, hivemind like behaviour, as they unwittingly summon a tsunami that will claim the outpost, and everyone in it.