Ethereal plane

The Ethereal plane was one of the planes of existence in various models of cosmology. "It is a plane out of phase. It is a place of ghosts and monsters. It is right next to you, and you don't even see it."

- Manual of the Planes

Ethereal Plane in the Great Wheel Model
In the 1st and 2nd edition D&D, as explained by the Great Wheel cosmology model, the Ethereal plane existed adjacent to the Prime Material Plane and connected it to the Inner planes (the Elemental planes plus the Energy planes). The Ethereal touched the Prime at all points through what was called the Border Ethereal. The non-Border region was called the Deep Ethereal. While in the Border Ethereal, a traveler could still see into the adjacent plane but only dimly and not very far, whereas those on the bordering plane could not see the traveler without magical detection. Verbal communication was not possible between the Border Ethereal and the bordered plane. The Ethereal was unique among the many planes in that an individual could exist in two planes simultaneously: the Border Ethereal and the adjacent plane.

When a traveler crossed into the Border Ethereal, she and all her possessions were converted to their Ethereal equivalents, metal became ethereal metal, flesh became ethereal flesh and so on, allowing free movement (in most cases) in any direction through the solid matter of the adjacent plane. Since everything was permeated with ethereality, an air-breathing creature could breathe ethereal air and could not drown in an ethereal lake nor be crushed by an ethereal rock. However, not all in the adjacent plane was insubstantial. Living things larger than one-celled animals generated an aura that radiated around them and prevented passage to their interior, so an ethereal traveler could not place a weapon inside a living creature where it would materialize and cause damage. A jungle would be an extremely torturous maze to navigate; it would be much easier to float above the vegetation or pass below its roots. Dense metals such as lead or gold also prevented passage of ethereal matter. And finally, some magic spells and alchemical mixtures could form an effective barrier.

Travel in the Ethereal and Border Ethereal was accomplished by force of will&mdash;you wished to go somewhere and you did, at your normal rate of movement. There was a sense of up and down but no real gravity existed. Objects released from possession would hover where they were dropped; it was impossible to fall in the Ethereal plane. To get to another plane, one had to pass through a curtain of vaporous color into the Deep Ethereal, then traverse that region until reaching the curtain that demarcated the Border Ethereal of the destination plane. Each Inner plane and demiplane had a curtain with a unique color. The Prime Material plane's curtain was turquoise. If the (usually magical) ethereal effect wore off while a traveler was in the Deep Ethereal, he would immediately be forced through a random curtain at a random location in the Border Ethereal and deposited on the plane which it bordered.

When you passed through a curtain into the Deep Ethereal, time slowed down to one tenth the rate it flowed in the Border Ethereal and the plane that it bordered. For every ten hours spent in the Deep Ethereal only one hour passed on the other side of the curtain. Metabolic and other natural processes slowed down also, so it did not feel like ten hours, when you crossed back through a curtain into a Border Ethereal you were only an hour older and an hour hungrier.

The Deep Ethereal swirled with large blobs of proto-matter (imagine a cosmic lava lamp) which could form a demiplane when a critical size was reached. Powerful wizards, technologists, or demigods could also bend the proto-matter to their will and create a demiplane. These nascent planes might exhibit some of the characteristics of the Inner planes or the Prime, but with their own rules of gravity, material make-up, etc., and even support life. Most demiplanes eventually collapse into themselves and break up or merge with another Inner or Prime Material plane.

Ethereal Plane in the World Tree Model
In the World Tree cosmology model, roughly coinciding with 3rd and 3.5 edition D&D, the concept of the Border Ethereal was dropped and the Ethereal plane became one of the transitive planes that coexisted with the Prime Material Plane. Like the Great Wheel version, ethereal creatures and travelers could see into the Prime, but not the other way around. Unlike the Great Wheel version, you could pass through solid objects, including living creatures, and you could hear the Material plane.

The Ethereal plane was mainly accessed by spells such as blink, etherealness, and ethereal jaunt. A phase door spell could be used to create a passage through the Ethereal, and Leomund's secret chest could temporarily stash a container in the Ethereal.

Certain creatures dwelled primarily on the Ethereal Plane. Examples of these include ghosts and ethereal marauders. Others can become ethereal virtually at will, such as thought slayers and phase spiders.

Ethereal Plane in the World Axis Model
In 4th edition D&D, according to the World Axis cosmology model, the Ethereal plane no longer exists; it was destroyed by the Spellplague.