The Pale Fisherman

At the bottom of Hollow Sea, this great beast slumbers. Knowledge of its presence has passed into legend, and even those stories are becoming forgotten in the towns at the edge of the sea. This creature resembles a colossal white leech, with long, ropy tendrils extending out from its segments. As it sleeps, the Pale Fisherman wedges itself into the chasms of rotten wood that make up the walls of Knotwood Deep. Here, in the eerie and black silence, the beast has escaped all discovery for ages.
When it wakes, the Pale Fisherman rises to the surface to feed. The fog above the sea is oppressive at the best of times, but when the great leech hunts, the fog smothers the town. Tendrils silently break the surface, and snake their way across the town. When these snares are set, soft bioluminescent nodules glow along the tendrils. When a curious person seeks out the source of the light, or an unlucky creature stumbles over a tendril, their doom is sealed. With a terrible contraction, the tendrils latch on with tiny barbed hooks and snap back, pulling the unfortunate soul under the water. If they are lucky, they drown before being vacuumed up in the Pale Fisherman's yawning maw. Fortunately for the people around the rim of Hollow Sea, the creature only fully surfaces to feed every few generations.
Where the Pale Fisherman's name comes from, however, is the last vestige of a warning told to the people of the towns.
Old sailors say that when the fog is thick, and you've lost your way back to port, you may come across another sailor calling for help from the mist. If you have your wits about you, you may notice that the call for help doesn't quite contain any real words. Or perhaps you'll notice the call is a little too resonant for a human-sized body. Or maybe just that the thick fog doesn't suppress the sound as much as it quiets other voices. If, the old sailors will whisper, you still haven't had the sense to turn away, you may notice the "lost" person has strangely smooth white skin. The tale differs on whether the mysterious person will be in a little rowboat, or whether they are treading water, but all versions warn that should you approach and try to help out this pale man, you'll be snared and pulled beneath the black waters.
In truth, this tale is the result of the Pale Fisherman's passive hunting method, when it needs a snack rather than a meal. The pale person is a lure, shaped roughly like a man in a slicker. Air pulsed in and out of tubes on the lure mimic the plaintive calls for help of someone stranded on the water. This lure is the last bit of a tale that people can pass down, a final warning of the true danger beneath the dark sea, until the Pale Fisherman rises once again for its next meal.