The Black Curtain
A ship that travels west or east eventually comes to the Black Curtain, a miles-high barrier of dark mist that stretches across the horizon and envelops the world like a death shroud. Ships can’t sail around the Black Curtain without coming into contact with the Frostfell to the north or the Eye of Io to the south.
Within the Black Curtain, vision (including darkvision) is limited to a few feet. This alone is enough to discourage timid seafarers, but some believe that the Black Curtain isn’t boundless—that it hides idyllic lands untouched by the wrath of Io. Many hopeful explorers have passed through the Black Curtain, hoping to come out the other side and see a vast, unclaimed paradise. None of them have ever returned.
The Black Curtain hasn’t always existed; it came into being some years after Io sank the continents, but its origin is unknown. It had already grown quite immense by the time the Arkhosian dynasties recovered from the worldwide devastation. Perhaps the most troubling thing about the Black Curtain is that it seems to be closing in. Islands on the edge of the black mist are gradually being swallowed up. It may take years, but the Black Curtain will eventually engulf the entire world. Consequently, many learned scholars have dedicated their lives to solving the mystery of this sinister phenomenon.