Beyond the Material Plane of Golarion lies a shifting realm of magic, dreams, and nightmares known as the
Æther. This “World Between” is a never-ending collection of planes tied together by
Ways, paths through the shifting planescape.
Travel to the Æther is often very difficult and dangerous for non-natives as unprepared visitors get lost in the strange landscape or slain by its otherworldly inhabitants. Even accessing the Æther typically requires potent magic or the willing aid of a native creature. For most inhabitants of Golarion, contact with the Æther is as rare as a Haunt manifesting or a wizard’s summoning.
There are, however, locations where the Veil between the Aether and Material world is thin. These Ways allow the influence of the Aether to bleed through into the Material plane and grants easier access to Fey and other Aetherial natives. Some notable examples are the Worldwound (where a gaping fester allows beings from the darkest Hells to boil into our world) the Land of the Linorm Kings (where ancient unworldly behemoths cross over into the frigid North) and the Darklands (where the sunless caverns plunge into some of the darkest reaches of the Aether.)
The Stolen Lands are one other unique example of Æther phenomena and is a host of a remarkable concentration of Ways to the point that some ancient studies by Ætherial scholars even refer to the Stolen Lands instead by the name of "Æther Vale." The unusual connection to the Æther is widely considered to be the explanation to the exceptional frequency of encounters with the fae within the Stolen Lands.
Æther Denizens: The Æther is home to a varied collection of otherworldly beings including outsiders, elementals, and fae.
Fey: The Æther is the native home of the fey. Hidden within the depths of the Æther, the two Fae courts of Summer and Winter are rumored to be ruled by Nobility that hold near godlike powers within their domain. The unaligned Wild Fae are also notoriously known as tricksters that might aid or doom travelers.
Other outsiders: All manner of outsiders from elementals to celestial and infernal beings can be found within the Æther. Ancient bindings and spells may be used to bring these beings forth to the Material world, but otherwise all outsiders require the magic of the Æther to survive.
Spirits: The spirits of the departed that haven’t been drawn to into the Boneyard can be found within the Æther.
Elves and Gnomes: Although distantly related to Fae, Gnomes and Elves have been reported to be viewed by the Fey similar to the way a human would view an ape. Some humans enjoy apes as pets and can even be trained to do tricks, but not the sort to bring to a Courtly function.
Gnomes: Gnomes were once residents of the Æther before some calamity or mass migration has drawn them into the Material plane. They still bear the shock of this departure in the form of the Bleaching, an affliction that affects the race and slowly ages them.
Elves / Drow: Elves and Drow are also distant relatives of the Summer and Winter fae (respectively) though they have adapted to the Material more effectively than the Gnomes. Often their dreams offer glimpses of walks through the Æther that were experienced by their ancestors. Their long past tie to the Æther is rumored to be the origin to their immunity to Sleep effects.
Æther and Magical transport:
Any Magical means of transport such as Plane Shift or Teleport are reflavored to interact with the Æther. This is largely for flavor and doesn’t have a large impact on actual gameplay, though a mishap might result in the unlucky mage getting misdirected in the Æther!
Other crunch changes:
Fae are treated as Outsiders and follow Outsider rules for spells and effects. Some Fae have existed on Golarion long enough to be considered Native Outsiders, though they require some contact with the Æther to survive.
Knowledge: Planes is replaced by Knowledge: Æther. Knowledge Æther identifies Fae, as opposed to Knowledge: Nature (although there’s enough Nature in the Stolen Lands that it’s still a valid skill to have.)
The Æther has variable planar effects (time, elemental, etc.) Legends abound of mortals trapped for centuries that seemed like days or realms that sapped the strength of those that venture into them.