Ethereal Plane

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

Description
The Ethereal Plane is filled with snow-like dust that floats in space and is colorless, having only black, white, and grey tones. While a creature is on the Ethereal Plane the plane they originated from is still visible to them in shades of grey. 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.

While walking around this overlapping plane, occasional chattering and distant chuckling of a weird alien laughter can be heard.

Denizens
This plane is filled with beings of drifting spiritual energy, that when passed through caused a sensation of one's blood freezing.
 * Translucent piranha-faced serpents that resemble strange looking angler fish.

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

Vox Machina's Visits to the Ethereal Plane
Vex'ahlia used Oil of Etherealness to transport 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.

The Mighty Nein's Interactions with the Ethereal Plane
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 it when Caduceus cast True Seeing on her, giving her Truesight but also making her seem as if she was high as she reached for ethereal creatures like elongated ghost fish swimming by. The ethereal fish seemed to be drawn by Marion Lavorre's singing.

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.