The Planar Fog

The Planar Fog is theorized space between the Material Plane and the Azural Sea, a layer of metaphysical (and perhaps literal) fog that obscures and represents the veil between worlds. Before the party learned to travel through it, it had only ever been hypothetical, something to discuss in Planar Philosophy 101. To get in and out of the Planar Fog, the party drew the door to their extradimensional haven on the floor of a wagon, then hitched the wagon to Amros' Nightmare. The Nightmare then galloped through planes into the Fog, taking the haven door (and by proxy the party) with her. 
This allows the party to move through space without being tracked or stopped by mundane threats such as bandits or monsters, but it has its risks. It is easy to get lost in the Fog, to lose sense of time and space. And while encounters with strange creatures and phenomena are rarer, they're all the stranger due to the nature of who can access the Fog. For example, the first time the party traveled this way, they were stopped by Morgana "Ana" la Fey.