The Maelstrom

The Maelstrom was a magical anomaly that occurred in the seas around Terragard and other island continents, cutting off all means of travel with the outside world. Supposed firsthand accounts of The Maelstrom revealed it to be one of the most violent storms you've ever seen, with ten times the lightning, plus magical energies raining from the sky in every form. Conjuring firestorms, raining acid, rays of frost, as well as teleportation anomalies tossing you about and teleporting huge amounts of water, ice, and earth. Some pockets without air, without sounds, or without anything, some ships sailing into pure nothingness and never returning."

Even teleportation magic and magical communication was cut off completely, and those who attempted to overcome this magical interference mostly met with disastrous failure.

There was one ship that was fitted to travel safely through the Maelstrom, made primarily of a magically resilient metal, including a shell that protects the crew and passengers from the dangers of the storm, but it never returned form one of its trips, though it was spotted many times, and supposedly change hands many times throughout the age of the Maelstrom.

The people of Terragard, now cut off from any aid or communication from their home countries and people, panicked and the political and social climate of the civilizations spoiled quickly, though much of this period of history was poorly recorded, and fewer records remain still after the Maelstrom cleared, but one thing is known: entire civilizations were wiped out, massive wars were fought and one civilization is suspected of destroying itself completely developing magical weapons to fight its enemies.

Researchers records from that time indicate that the suspect was a weakening between the planes causing the elemental chaos to leak into the mortal plane, but something like that would require a massive shift of power between the divines, enough to destroy or create several Gods.