Since time immemorial, Sundland has been surrounded by a ring of storms, which battered and sunk any vessel seeking to enter or leave the oceans around the island subcontinent. The legends say that the Old Gods put the Stormwall in place to prevent mortals from gaining access to the secrets and treasures held within the island, and that only Helikhar's direct intervention allowed the dwarves to travel there via Gate during the Godswar.
Regardless of its origin, though, the Stormwall protected Sundland, and later the Sunderwyl Republic, from the turbulence of Continental politics and warfare. The Churnings kept the Sundalfolk in touch with the rest of mortalkind, as Sundal Veilwalkers met others in Veilguard and brought back new ideas and goods from the outside world back with them, but other than Churnings the Sundalfolk were able to mostly ignore the outside world.
That imposed isolation was first truly broken 1100 years ago with the Wylding Migration. The Winter Fae seers of the Wylding refugees foresaw a once-in-a-millennia break in the storms surrounding the island. They sailed their fleet of cargo vessels through the gap, away from the tyranny of the God-King and onto the shores of Sundland.
Folk tried to find gaps in the Stormwall in the decades afterward, seeking some rhyme or reason in the storm's behavior, but none were successful. While the Republic was happy to take in the few survivors of those attempts, the general consensus of the nation was that the Stormwall was a net benefit, keeping the chaotic and often violent folk of the rest of the Continent at bay even if it did impair their ability to bring new goods and ideas in.
Then, three centuries ago, the Leviathan Churning began, and cultist forces somehow managed to sail an entire armada of ships into the seas inside of the Stormwall; how they did it, nobody is sure, but once they arrived they started wreaking havoc on the coastal settlements of the island and swiftly began encroaching into the interior. The Republic needed help, but couldn't bring in any fleets to aid them with the Stormwall in place. Veilwalkers threw themselves into the task of finding ways to disperse or clear the storm, and finally, with the assistance of some brave Delvers in the depths of the Golden Sea, found one of the keys to the Stormwall's persistence: a shackle, binding a great Spirit of storm and wrath. With the aid of a Herald of Xyston, the Veilwalkers forged "Chainbreaker", a mighty Xenomalleum that they used to shatter the Spirit's bindings. Freed from its millennia of imprisonment, the grateful Spirit turned its wrath upon the cultist fleet and sunk a full score of ships before it eventually returned to the Elemental Sea.
With this, the Stormwall was weakened, but not gone - more shackles remained, and the Veilwalkers frantically sought out more within the Worldtooth Mountains, eventually finding and shattering the rune-forged cage deep within that Labyrinth that bound a Spirit of destruction to the storm's winds. Attempts were made to find the shackle that presumably still exists within the Calcific Isles, but none were successful. It proved unnecessary, in any case, as the Stormwall's destructive power was greatly weakened with two of the bound Spirits freed, and a fleet of vessels from the rest of the Continent soon arrived on Sundland's shores to hunt down and crush the cultists' ships.
In modern times, the Stormwall still exists, but more as a boundary marker than anything else: a ring of stormy seas and inclement weather that surrounds the Republic, easily passed through by experienced sailors with sturdy ships. With the clearing of the magical storm, Sunderwyl has become another part of the trade networks of the Continent, and has had to contend with new immigrants and cultural influences. Fortunately, the Republic's welcoming and can-do attitude has proved more than up for the task of a brave new world without the Stormwall's insulating presence, thriving upon the new people, goods, and ideas that the Stormwall's absence has brought to their shores.