The Ethereal Plane

The Ethereal Plane has become a focus for our heroes, following the revelation that the vines they're transporting hailed from that plane, and must be returned to it.

So far as our heroes know, the Ethereal Plane is divided into two parts: the Border Ethereal, which overlaps with the Material Plane, and the Deep Ethereal, which is a world of its own. They know that the Ethereal Plane is a realm of ghosts, and that the Deep Ethereal is more a realm of thought than flesh — travel there takes less time than it does on the Material Plane, and hunger and thirst are lessened.

Short of high-level magic, the only way our heroes have been told of reaching the Ethereal Plane is to find a portal there, which takes a considerable amount of luck and patience, as there are no known stable portals to that plane near where they are.