The Ertolian Caverns are a network of submerged underwater caverns found in the swamplands of the Kingdom of March. Legend says that the caves hide an ancient species of sea-elves called the Füledhin who have been hiding for over a millennia, ever since the Conquest of Thaelia.
Every year, fearless adventurers dive the caves in search for treasure or signs of the Füledhin. Only a few surface back up, either empty handed or holding gear lost by previous explorers.
Despite being located on the country's swamplands and frequently being contaminated by the swamp's murky discharge, the cave's water is always clear. Scholars and clerics have studied the water in recent years for traces of magic or divine blessing but have found nothing that could explain it's properties.
This entity is written for the Describe the encounter worldbuilding prompt.