The Fae Ethereal is the realm that separates the Realms of the Fae from the mortal realms.
The realm appears as a simulacrum of the mortal realms but with distinct changes that reflect the manner and appearance of the Fae realm. For non Fae the realm appears very misty and vision is reduced to a few feet.
This borderland is inimicable to iron and any cold iron brought in will rust and decay within hours.
Entry to the Fae Ethereal is only possible from the Mortal realms through weak spots that often manifest as circles of mushrooms known as Fairy Rings. The Illusionist Maze spell may take you to the Fae Ethereal as well, depending on the region and influences.
For mortals this realm can be very dangerous as the Fae hunt the mortal beasts that stumble in through the Fairy Rings and become lost and tired. Once the mortal creature falls asleep they are quickly dispatched by the Fae.