The Crescent Sea
The world of the Crescent Sea is plane-locked to players for all intents and purposes. Beings cannot enter or leave the plane via standards methods of inter-planar travel. The major exception to this plane lock is the appearances in the city of Tearnyn.
Localized demiplanes exist, allowing most spells to function as written. Banishing something, for example, sends it briefly to a small temporary demiplane. Pocket demiplanes representing the elemental planes exist within the plane-lock. Parallel planes like the Plane of Shadows and the Ethereal Plane exist within the plane-lock as far as they lie parallel to the world. They cannot be used to exit the plane-lock.