Charlatan in Mud

Each spring, snowmelt from the Ashwind creates waterfalls of feathery mist that drape the hills below in clouds of silver vapor. Under one such waterfall, a rugged hillside shelters a dark secret in a pool hidden from wind and sun. The water bubbles, and shapes emerge from mist and shadows - haunting illusions that can drive travelers mad if they linger too long. A crude wooden sign nailed to a post reads, "Turn back. Cursed."
Deeper: From summer through winter, the hollow is quiet with no trace of malice. The water in the pool is gone, reduced to damp mud, tacky and viscous like a wet dough. There is no trace of the sign.
[Spring] The water in the pool is black, its surface swirling with an iridescent slick. Mists from the waterfall, shift and move creating ripples and curls like air currents of some invisible entity. Many believe these are the ghost of people who went missing here. Others believe they are malicious denizens from the world beyond.
In truth, the sign, the invisible entities, the oil slicked black water are all illusions conjured by the Charlatan Clam also found below the Troll Bridge [C-38] which slumbers through winter here in the rich mud each year.
Its spring time awakening is an exhausting process of feeding and remembering, touching scrying pearls it has hidden throughout the mountains and placating the great denizens lest they choose to retract their favors from the ancient mollusk. Before long, it will have regained enough strength to change its form and travel back into the Ashwind mountains where it will plunder and feed properly.
[Discovered] But in this weakened state, it may be bargained with if discovered. It has a wealth of knowledge and caches of treasure hidden just about everywhere water pools in these hills and mountains.