Dark Waters Rising

From the publisher's website:

A Pathfinder Roleplaying Game compatible adventure for 5th-level PCs by Ron Lundeen

Catastrophe strikes the frontier village of Swallowfeld! With a grinding groan, the town’s mill slews into the Kilian River and breaks through the ceiling of an ancient subterranean dungeon. This accident frees a long-imprisoned evil to prey upon the shocked townsfolk. When several Swallowfeld residents—some innocent and some not so innocent—are spirited away into the rapidly flooding dungeon, it falls to a brave group of heroes to venture underground and rescue the missing before dark, rising waters seal their fate.