Maelstrom

The Maelstrom is a terror of the Inner Sea. It steers clear of land, but wanders, mostly the southern half.  Unlike a normal storm it gives no clue of it's approach, stalking and pouncing upon hapless ships.  It howls and screams, thrashing the sea and tearing at ships and crew.  If yer lucky, very lucky, you survive and pass from the storm and nothing worse befalls ye.  Some would say the next most lucky die, drowned and lost at sea in sunken ships. But at least still in our world.  Then there are those that said they passed the storm but found themselves elsewhere.  A place they called the Cursed isles.  It is encircled by the storm.  Monsters and horrors dwell in land and sea.  Treasures to be sure, but few find their way back to our world. 

As told by a sea captain in the Shard Port of Taipinnan

for the past 557 years the Maelstrom has stormed across the Inner Sea. It has mostly kept to the southern part, but occasionally venturing into the north.  The precise cause is unknown - be it a Warp Storms, an act of magical enchantment, The Sidereal Fae or some ancient Gods. It never ventures onto land and has no preceding signs.  It simply appears within hours of a ship and sweeps across the sea as if it were a creature hunting prey.  If it is in some way has a purpose and is seeking something, none knows what that is.  Some ships are destroyed. Some survive and some pass through the Maelstrom to another world.
The Cursed Isles exist beyond the Maelstrom and are encircled by it.  The Isles are a multitude of islands surrounded by seas with normal weather, days and nights. The Isles are inhabited by creatures and survivors from our world, and possibly others. It is unknown how many have attempted to escape by sailing into the encircling Maelstrom, only that a few have escaped to bring wild tales to the real worl.