A level 10 adventure for 5E

The village of Drydale has a problem. People are disappearing in the night. Worse, some return bearing the scars of a horrific surgical procedure and possess strange new powers. These unfortunates have no memory of their ordeal beyond a terrible vision of drowning in the dark. The adventurers arrive in time to stop a group of icthyoid monstrosities from abducting Drydale’s blacksmith, and then track the creatures back to a network of caverns. There they face a host of enslaved minions and uncover an otherworldly evil lurking in the black waters beneath the mountain. Will the heroes end the threat to Drydale? Can they avoid eternal servitude in the drowning caverns of the fish god?