Delthur's Folly (5E)

From publisher blurb: Hidden in the wild uplands beyond the Selka River, the sinkhole-mine of Delthur’s Folly is a byword among the locals for a fool’s errand doomed to fail. The only survivor of the fated expedition — driven mad by his ordeal and raving of “the water’s sibilant whispers” — died shortly after staggering into Dunstone a year or more after Delthur and his miners disappeared into the wilderness. And yet, every year adventurers cross the Selka in search of the mine — gold and silver are said to lie within. Some return empty-handed and some do not return at all. Local legend has it that something terrible dwells in the mine.