The Worldmists

The billowing wall of fog loomed everpresent throughout the journey, towering over our three-masted vessel as a mountain giant might over a gnome. I was of two minds the entire time: part of me, that voice of adventure and discovery, wanted to request the captain to sail just within the mists, just enough that I could say I looked within them; the much larger part of me whispered urgently of danger, taking all my will to keep from jumping overboard simply to swim directly away from those mysterious depths.
It bears specifically noting for the sake of the suicidally-curious that to the west, north, and south of the known edges of the map a barrier to exploration exists. Visible from several dozen kilometers away, the Worldmists tower imposingly. Vertical expanses of thick white fog, they climb so high into the sky that even Emporium vessels dare not attempt to fly over them. No one knows what hides within, nor by what magic or mechanism they are maintained.
On rare occasions, a suicidal brave crew (usually consisting mostly of gnomes) will sail boldly into the mists, determined to uncover their mysteries. None of these crews have ever been heard from again, though two notable attempts did see wreckage drift out from the mists to ships waiting for contact. What flotsam was collected showed precise slashes, as though some enormous chef had cut through the hull materials of the missing vessels in a single clean stroke.
For the most part it is safe to sail just within the wall of fog briefly, but most sailors hold that doing so invites disaster, cursing the ship that does. This is perhaps not entirely superstition; all who have remained within close range of the wall for any length of time agree the phenomenon inspires an instinctive dread, much like knowing a large predator lurks just beyond one's camp.
It should be noted that kraken and other creatures of the lightless ocean depths are encountered in slightly-greater frequency near the Worldmists; most captains that sail near them do so only briefly, usually following one of the major ocean currents to shave time off their trip.