The Transitional Planes

The Transitional, or Transitive, Planes are all Pathways from one plane to another, which itself is a proper plane, in the sense that you can actually be there. This is, of course, opposed to Pathways where you cannot exist or has no space, such as a Portal. These are loosely grouped into "Inner Transitionals," "Outer Transitionals," and "Intermediary Transitionals," though is often supplemented with the "Universal Transitionals," technically a subset of the Intermediaries.

Elemental Chaos
The Elemental Chaos connects the many specific Elemental Planes, which act as special "Nodes" of elemental energy which bleed outward, interacting with eachother within this Elemental Chaos.

The Boneyard
A vast graveyard of infinite proportions, often believed the be the resting place of every forgotten corpse across all of Nexus. It connects the Shadowfell with several Outer Planes and the Astral itself.

The World Tree
An incredible, seemingly-infinite tree whose trunk is hollow. Fates live beneath it's roots, and it's branches reach through existence, connecting many planes of existence. The Tree's branches do not reach Ostoria, owing to an ancient argument between the Fates of the Tree and the Fates of the Giants. While the tree's canopy is close to the Elemental Plane of Air, it does not truly reach the plane, also owing to a disagreement between the Tree's Fates and the Fates which live alongside the Djinn.

The River Oceanus
Oceanus is a massive river which loops into itself and the River Styx. Coming from the Delta Aurus, it flows around and through several planes, such that Arborea, both layers of Bytopia, one layer of Elysium, the base of Mount Celestia, and The Beastlands are all planes scattered across it's twisting shores. Beyond these, the river loops around the roots of The World Tree, and in turn touch Ysgard, before flowing into the Gate Plane Delta Ferra. It's waters are inhabited by many creatures, ranging from dragons and giants to many celestials, particularly many Kirin.

The River Styx
Styx is the other incredible planar river, also looping back into the River Oceanus. It starts in the Gate Plane of Delta Ferra, the Stygian Delta, then flows through Acheron, 3 of the 9 layers of Hell, through the Prison Gateway of Carceri, through the heart of Gehenna and it's many layers, around the Deathway of Hades. The Deathway splits the Styx into 9 rivers, 8 of which then spiral through the infinite layers of the Abyss and through Pandemonium, before reuniting with the Styx proper, and everything flows back into the Delta Aurus.

The Styx is home to many fiends, the Ferrymen being one of the most famous, but not much else. This is because the Styx is the main battleground of the Blood War, and while many things are happy to continue the eternal fight, most mortal and mundane creatures are not in such a mood.