Ethereal plane

The Ethereal plane was one of the planes of existence in various models of cosmology.

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 (also called the Elemental 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. 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, he and all his possessions were converted to their Ethereal equivalents, metal became ethereal metal, flesh became ethereal flesh, and so on. Thus an air-breathing creature could breathe ethereal air. Since everything was permeated with ethereality you could not drown in an ethereal lake nor be crushed by an ethereal rock. However, not everything was insubstantial. Living things larger than one-celled animals generated an aura that radiated around them and prevented passage to their interior (so a traveler cannot materialize inside a wall for example). 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.

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.

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 coexists with the Prime. Ethereal creatures can see into the Prime, but not the other way around. It is mainly accessed by spells such as etherealness and ethereal jaunt.

Certain creatures primarily dwell 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.

The Ethereal Plane was destroyed by the Spellplague.