The Shattered Sea
The Shattering is not exactly geography, but more a violation of geography. For several months prior to the Long Night, there was an unprecedented storm raging in the eastern ocean. It blocked passage and even swallowed the last treasure fleet returning to Aeridor. Many lives were lost and sailors feared the passage may never be open again.
And then there was the Long Night, when the Waystones were shattered and alien forces invaded the Hesperean Sea and unleashed untold horrors for eight sunless days. Naught was heard from the Old World and again people feared they would be abandoned.
When dawn finally came slim hope returned. But then people slowly became aware that they were truly alone. Ranging ships came upon walls of mist that could not be penetrated. Magical communications that had been set up to Old World magicians reached no one. Those few with the power to teleport without the Waystones could not focus their magic to such great distances. Even the secret ways of the planes were blocked— there was no way to reach the world they had come from.
The gods themselves are not (or choose not to be) able to directly intervene on the Hesperean. It is widely debated whether the Shattered Sea was spared a fate that befell the Old World or if the sea was severed to save them from the Baorn.