Ethereal Plane

The Ethereal Plane is one of the Planes of Existence that overlaps the Material Plane.

Description
The Ethereal Plane is filled with colorless, snow-like dust that floats in space and appears only in shades of gray. While a creature is on the Ethereal Plane, the plane they originated from is still visible to them in grayscale. They cannot interact with anything that is not on the Ethereal Plane, nor can anything not on the Ethereal Plane interact with or see them.

Skein is a drug which grants those who ingest it the ability to perceive the Ethereal Plane. Those who take it can hear occasional chattering and distant alien laughter.

Denizens
This plane is filled with beings of drifting spiritual energy, such as translucent piranha-faced serpents that resemble strange looking angler fish. Passing through these beings while aware of them causes a sensation of one's blood freezing.

Phase spiders travel between the Material and Ethereal planes at will. Night hags, succubi, and incubi can also do so, and they use this ability to reach mortals in their sleeping chambers and influence their dreams.

Campaign 1: Vox Machina
Vox Machina retrived a vial of Oil of Etherealness from the Umbrasyl's lair. Later, Vex'ahlia applied it to herself transporting herself to the Ethereal Plane. While there she passed through rock to find the Sword of Kas buried in Thar Amphala. After the effects of the oil wore off, Vex and Keyleth recovered the sword.

Campaign 2: The Mighty Nein
Beauregard and Mollymauk Tealeaf were temporarily able to see into the Ethereal Plane while under the effects of the fungal drug Skein. Beauregard also saw into the Ethereal Plane when Caduceus cast True Seeing on her, and she was able to perceive creatures resembling elongated ghostly fish swimming by while she had Truesight. The ethereal fish seemed to be drawn by Marion Lavorre's singing. When Caduceus cast True Seeing on himself on Rumblecusp, however, he did not see any ethereal fish near the volcano.

Fjord and Jester Lavorre are both able to access the Ethereal Plane for one minute using the spell Blink. Jester notably used the spell to escape from the temple of The Platinum Dragon in Zadash when she was caught impersonating the Herald of the House. Fjord and Jester both used the spell to escape from Thelashas, the blue dragon in Halas's extra-dimensional manor.