The setting looks a bit like this
The last book is set in a valley high in a Northern Irish mountain range. In the valley is a lake with a village by its shore. The village is the opposite of Ballydog. Their community is a utopia, everyone is sweetness and light. The citizens are all blessed with long and contented lives. But they have a secret, revealed early in the plot. The lake is home to a monster.
Here are some titles I like, although not quite enough:
Last Days of LegendYou could build on any of the above or be original. No suggestion too crazy. There’s a prize for the winner. Not sure what yet.
Trial by Water
The Last Days of Lough Linger
Lost Soul Lake