The Ordering of Heroda

Imagine a pearl. Layers upon layers of smooth nacre build up around an irritant that helps protect the soft defenseless tissue of the oyster, developing a beautiful gem over time. Now imagine instead that the pearl was protecting itself from the oyster with each layer of existence creating an additional buffer between it and the caustic alien world inside the oyster. This is the world of Heroda, one of millions of worlds that flicker like guttering candle-flame in the alien otherness of the Far Realms. Worlds form and collapse in the Far Realms all the time, examples of failed worlds rise and set above Heroda every night in the form of the two moons Pandom and Seles.
Formed around detritus from the inner planes, thrown into the Far Realms by some divine act, dreadful cataclysm or random accident, the core of a Pearl World is hidden deep within the layered reality of the world. This kernel of persistent reality provides the core around which a new world will form because the reality of the Cosmic Wheel is the antithesis to that of the Far Realms. It is unknown what forms the core of Heroda but it has a strong attachement to reality, its secret lies beneath or is forming the sea of molten iron at the bottom edge of the Sphere of Fire.
Reality in this way is always under attack, though attack is not quite the term. Giving form and function to the formless and functionless misattributes the dynamic. It is better to say that the worlds in the Far Realms are under a constant unyielding pressure and under this pressure their reality only grows stronger...until there is a crack. Though many scholars disagree with this, it is the prevailing theory by a hair among those learned and studied in the phenomenon. The influence of this "Hardened Reality" is called a Reality Shadowand all stable worlds in the Far Realm have this.
On the world of Heroda, gods and spirits have risen from the primordial soup of the Spheres of Existence, swimming the seas of possibility since the first glimmer of stability and sanity took hold upon the world. Invaders from beyond came as well, tendrils of great creatures from the Far Realms or strange intrusions of the maddening, shifting chaos itself striding out upon the world. It is at this stage that most Pearl Worlds fade, dissolving into the endless ephemeral layers of the Far Realms. Where Heroda and other worlds like it differ is that their Reality is enforced so strongly that instead of the eldritch devouring them, they absorb and enforce the worlds reality on the eldritch.
Many powerful beings, spirits and gods that govern the world today were once intrusion of the nether, creatures of the void who instead became trapped and changed to something which would be able to exist properly within the framework of reality on the world. The elven gods all stem from the Tree of Solitude, a sphere spanning tree which is the remnant of a cyclopean tentacle of some elder being that was shorn off by the pressures of the spheres healing the wound it caused during its entry. The god Solag rose from a shard of the Sphere of Earth ejected from the world by that event, landing back on the Material Sphere, infused with eldritch energies and carved by elves effected by its madness. As many gods and spirits arose from these intrusions in the time before the Mythic Era as did those that rose from the primordial soup of Heroda itself.
The Spheres of Existence rise from the core in layers that help protect the from the Far Realms. They rise from and are anchored to the piece of detritus from which they were originally formed but each sphere has its own rules that do not necessarily follow that of the other spheres. The Mortal Sphere sits at a sort of Goldilocks zone of Reality where survival for the mortal races is at its easiest. The spheres have a specific order to them, though this can change through events known as "Upwells" where a portion of one sphere intrudes into or through another. This almost never goes beyond the next sphere in order. As a rule, living is easier for Mortal Races when moving "Up" through the spheres, which is Life, Water, Air. Moving down from the Mortal Sphere would entail moving through Death, Earth and Fire, the least habitable spheres for most mortal races. On Heroda the Astral and the Ethereal behave nearly the same as on the Great Wheel but are far more dangerous as they occupy a space that allows the madness of the Far Realms to intrude very easily. Travelling through the Astral or the Ethereal can be the quickest way to move between the spheres as it allows you to ignore the restrictions of a spheres locations relative to your own but the chance that you might be devoured by some astral monstrosity or find yourself in a place far from where you intended due to some eldritch intrusion is so high that most that can use those methods do not. 
 
The Astral If the Ethereal adheres to the mortal sphere like a skin, the Astral fills the metaphysical void between the spheres like a thick fog of unfettered and terrifying potential. 
The Ethereal This plane layers against the Mortal Sphere like a skin and as long as one stays to the border ethereal, travel here is relatively safe. 
Sphere of Fire The Sphere of Fire is the bottommost sphere. A sea of fire for as far as the eye can see.
Sphere of Earth This sphere is a thick, endless depth of earth and rock. Crystals and pure minerals are found everywhere here and as a general rule there is not any oxygen aside from pockets trapped in caverns. 
Sphere of Death Death is the inverse of life and it is against the mortal sphere that the spheres of death and life are linked eternally. A strange place, it is a dark mirror of the Mortal Sphere and several gods keep the eternal resting places of their followers here. Equally able to be paradise and drab, undead filled hell the Sphere of Death is the first level down from the Mortal Sphere and is the first taste of the danger that descending represents. 
The Mortal Sphere The home of the mortal races of Heroda, this is the centermost sphere of Heroda and is the most stable of the spheres. 
Sphere of Life A sphere that loosely correlates to the Material Sphere but it is nature and life taken to the extreme. This is where the Fey of Heroda make their home in a place called the Dreamfall.
Sphere of Water Water is the ultimate solvent for life as we know it and the Sphere of Water is as rich with life as the Sphere of Life, just different. Vast forests of kelp and coral cover the distant floor while the surface where the sphere touches that of the Sphere of Air is an enormous ocean dotted with the rare island of elemental earth flung there over the centuries. A vast majority of the Sphere of Water is just that, endless water with no end or any sense of what is up or down. 
Sphere of Air An endless expanse of air populated only by drifting pieces of other spheres, the most common being that of the spheres of Earth and Mortal but also some from Death and Water. Aerial species thrive here and it is one of the most accessible spheres for those native to the Mortal Sphere. 
Pandom The large red moon that orbits the world is the remnants of a separate world that has been able to anchor its reality to Heroda. Pandoms attachment to the Great Wheel came before it found itself orbiting Heroda.
Seles A gleaming white sphere, it appeared in the skies above Heroda long before its dire twin Pandom. Broad plains of silvery grass sweep across chalky white soil stopping only where jagged mountains reach almost out of the atmosphere. All of this is perched on sheer shelfs of land that floats above seas of molten platinum. Reality on Seles was extraordinarily weak before it was moved into the reality shadow of Heroda.