The Great Beyond is the multiverse of planes unique to Golarion, the setting of Pathfinder. In many ways, it is a mash-up of ideas taken from both the Great Wheel which preceded it and its World Axis competitor. There are nineteen major planes that comprise the Great Beyond, modeled as two spheres: the Inner Sphere and the Outer Sphere. Connecting the two spheres is the Astral Plane. The spaces between the primary nineteen planes contain countless demiplanes.
The Planar Adventures splatbook describes the cosmology more like an onion of layers with the material plane acting as its core, with each layer corresponding to a metaphysical distance outwards from the material plane. Remember that the planes exist on dimensions incomprehensible to mortals, thus these terms are only used to represent the planes in two or three dimensions.
The Inner Sphere surrounds the Material Plane and consists of the six basic essences that make up reality. It is depicted as consisting of the four Elemental Planes, with the Energy Planes of positive and negative acting similarly to spiritual versions of the North and South poles. The First World; a "rough draft" universe populated by Fey creatures and proto-deific entities called the Eldest, exists between the Material Plane and the Positive Energy Plane. Opposite to the First World is the Shadow Plane, which has relative proximity to the Negative Energy Plane; the Shadow Plane mirrors the Material Plane and acts as a transitive plane, but thankfully acts as a buffer against worst influences of the Negative Plane itself. The Ethereal Plane is a medium that insulates and touches upon all of the Inner Sphere planes, serving as the Transitive Plane .
The Outer Sphere surrounds the Inner Sphere and is composed of a seventh basic essence of reality called Quintessence, which is the material of moral and ethical philosophy and can take an infinite number of shapes and states, allowing for the creation of the outer planes when it reacts with thoughts and beliefs. When Quintessence interacts with a mortal soul that soul becomes an outsider and takes its place in the Outer Planes, with one potential eventuality being that they merge with the plane. The Outer Sphere is depicted as consisting of 9 alignment based planes, all insulated by the Astral Plane as a Transitive Plane. Most of these planes are virtually identical to their D&D Great Wheel counterparts; "Heaven" is conceived of as a mountain consisting of seven ascending layers while "Hell" consists of Nine descending layers populated by Devils, though these themes were not entirely unique to D&D or Pathfinder in the first place. Additionally, the various "in-between/border" planes have been removed for simplicity, though several of the Pathfinder planes are synonyms with what were historically their neighbours in the Great Wheel (for instance, the Abyss is also called Pandemonium, while Abaddon is alternatively called Gehenna or Hades) Also, thanks to differences in the way the divinities are spread out and their interactions with mortal souls; other planes are unique to the Pathfinder setting, such as Boneyard and Axis although certain thematic cues such as impossibly tall spires and cities of portals still exist. The Outer Planes are divided into the Middle Planes of Axis, Boneyard and Maelstrom, the Upper Planes of Elysium, Heaven and Nirvana, and the Lower Planes of Abaddon, The Abyss and Hell.
Scattered throughout the cosmology are multiple demiplanes and dimensions, which defy neat categorization.
What lies "outside" the Outer Sphere is only speculated within the setting. The limbo of Maelstrom is largely considered to be the "outermost" Outer Plane, acting as the shell that surrounds EVERYTHING both literally and metaphysically; with its entropic influence eroding the quintessence of all the other planes and recycling it back to the Positive Energy plane of the Inner Sphere where it can be used again to create new matter. However, the existence of the Abyss supposedly represents the cracks in this outer shell, with the deepest rifts eventually leading "outside" to a void of nothingness where time and space have no meaning. Some scholars further speculate that the sphere of the multiverse is just one sphere in a multitude of multiverses which co-inhabit the endless void, but are unreachable due to the impossible distances and unknowable forces.
|Great Beyond of Pathfinder|
|Astral Plane Ethereal Plane|
|First World Inner Planes Shadow Plane|