The Ethereal Plane

The Ethereal Plane is one of the transitive planes, which connects all planes together. Even other Transitive Planes, such as the Astral or Elemental Chaos, or more specific transitive planes, like the River Styx.

The Ethereal Plane, despite being one of the most well-known, is also one of the strangest. Unlike other transitive planes, the Ethereal experiences no "bleed" from other planes, and also unlike other transitive planes, the Ethereal is a Layered Plane. These two facts tie together, because the Ethereal's layers are what prevent the bleed. Essentially, where the Ethereal meets another plane, it conforms to the orientation and nature of the plane it is touching in what is called the Border Ethereal. Within this border, the connected Plane and the Ethereal plane are very close, and beyond being close they also share the majority of their properties, including the shape of the terrain and structures. Generally, any Huge or larger object which is anchored to the terrain also carries into the Border ethereal.

The difference comes from the fact that the Ethereal Plane's properties still apply within the Borders, mostly anyway. The physical is not; Only creatures are solid in the Ethereal, everything else is merely pseudosolid and can be moved through as though difficult terrain. Gravity also doesn't fully function, and any creature can levitate or fly within the Ethereal, including the borders.

The Ethereal has one additional quirk, as a transitive plane. It has truly native creatures. Creatures not native to any other plane. The Spirits. The spirits are strange primal beings which are a sort of mirror to those things that live; More specifically, it is the spiritual reflection of a creature's soul. Soulless creatures, then, do not have a spiritual reflection.