The Dream Serpent of the Atherion Sea

Sailors tell of a serpent so vast it spans whole leagues of the sea, able to appear in the dreams of mortals. It comes to those on the verge of great change, either as a guide or a tormentor, and can grant visions of the future or drive a person mad with endless nightmares. Many believe the Dream Serpent holds the knowledge of the Sunken City of Avalore, and that those who dream of it may be fated to uncover the lost city's secrets, or be drowned in their quest.

Some say that if you dream of the serpent you must never look it in the eyes, for doing so will bind you to the sea. Those who dream of it three nights in a row are destined never to return from the Atherion's depths.

Sailors who dream of the serpent will not sail for three days, and tell no one but a priest.
A coin under the tongue is said to keep the serpent from following a dreamer onto the waking sea.
Cartographers mark no route to Avalore. To chart it, they say, is to be dreamt of.