From ‘Introduction to the Planes,’ by T. Ketzvih:
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THE ELEMENTAL REALMS
The Elemental Realms are characterized by an overwhelming predominance of a single element, save for elemental ‘pockets’ of clumped-together material of a different element, but are technically defined by the following criteria:
1- Any substance not a part of the plane’s predominant material is attracted to other foreign material across the entire plane. The exact mechanism wherein this happens is unknown, but is directly tied to the mass of the material and is not simply a surface phenomenon like on the Mortal realms.
2- If sufficient foreign matter (that is, material not part of the realm’s primary element) is present, it will spontaneously generate more matter of the realm’s type in a process known as Genesis. While insignificant in comparison to the infinity of the plane’s existence, it does serve as one of the creation points for their expansion.
3- Elementals will spontaneously come into existence within a corresponding plane of existence, animate masses of a single element (contrast to mortal life, which is a combination of multiple distinct elements). Some speculate that an elemental is actually a motive spirit which merely inhabits local matter, but such discussion is beyond the scope of this work.
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Water
The Realms of Water are predominantly seen as an eternal ocean with neither surface or bottom, though as the realm abuts other planes this naturally becomes less true. Though the exact nature of the transitionary planes between elemental Realms is beyond the scope of this primer, further details may be found in the relevant realm’s dedicated chapters.
Even within Water proper, world-sized bubbles containing drifting bits of stone are scattered throughout the depths, and serve as the primary bastions of non-aquatic life on the plane. Free-floating plants serve as an abundant food source for truly massive behemoths, some of the largest creatures in any realm making their home in some of the emptier portions of the realm and planes.
Illumination comes from either a false sunlight filtering through the waters, luminescent algae permeating the waters, the water itself glowing slightly at all times, or from luminous crystals functioning almost as a sun in the sections of waters where they are found.
Genesis for Water is most common with stone and rock, in that a spring- usually tiny, but can be massive- forms and eternally releases water. A similar process occurs in air or other substances, but predominantly takes the form of mist or steam, allowing moisture to slowly accumulate; increasing humidity and potentially causing rain or snow.
Venturing into the depths of Water itself is of course fatal; fierce currents of water coupled with strong water quickly serve to rip apart the flesh of any who venture into the metaphorical depths of the realm.
Literal depth is of course fine, the lack of water pressure within most of the realms ensuring no creature is crushed under infinite tons of water (save for, of course, the Abyssal Depths).
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Earth
The Realm of Earth appears to be an almost solid mass of dirt and stone, the interior of some eternal mountain or continent. Within Earth exists a wide array of stone, metal, and minerals of all sorts. Mountain-sized gems, the most fertile grounds in the multiverse, veins of precious metal the size of entire worlds, crystal caverns glowing with otherworldly light, rivers of molten rock, lightless deserts of white sand, and more besides.
Of all the elemental realms, Earth is the one which has the most civilization within it; thanks in large part to the immense amounts of building material at hand. Enormous cities, carved from rock and illuminated by radiant crystals, sprawl along the edges of the largest caverns and cracks, taking advantage of the gravity the pockets provide to give a solidity and stability to the constructions.
Traveling too far into Earth’s depths is perhaps not technically fatal- spending excess time petrifies all non-earth materials, of course, leaving a statue of whatever previously existed- but investigations into how long a soul remains ‘alive’ when preserved in such a manner are inconclusive.
Genesis within Earth holds immense variation within the different substances which could trigger it, but is among the most sensitive to begin. Even a tiny amount of water will continually precipitate sand into whatever container or body of water is is a part of; air will grow dusty; fire will produce massive piles of ash which swiftly decompose into dirt; radiance will cause crystals to grow; void creates and enlarges stone; and shadow often creates metals. This is, of course, something of an oversimplification and nearly any form of earth can be created as a result of different kinds of Genesis, though there exist numerous research projects (many ongoing) as to how to create specific results.
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Fire
The planes of Fire are among the most immediately hazardous to non-natives. Anything unprotected immediately erupts into flames, and even in its pockets temperatures are still substantially higher than all but the hottest places within the cosmos. Closer to the edges of the realm, eternal wastelands of fire and ash, littered with volcanoes and eternally raging wildfires, are significantly more common. While life (even non-elemental life) thrives within all but the absolute deepest portions of Fire, it is by necessity almost entirely non-flammable and therefore bears little resemblance to the biological makeup of the other planes.
It still has its wealth of civilization, of course. Some of the cooler pockets are almost bearable to foreign life, and even for those which aren’t, heat regulation enchantments are a relatively simple matter.
Within the deepest portions of Fire, anything and everything begins to spontaneously combust, even the enchantments used to prevent that very fate. A similar fate befalls any amount of matter sufficient to ignite Genesis; while the resulting conflagration rarely outright consumes whatever triggered it, it quite often leaves it as either a burned-out husk or an eternally burning star among infinite other flames.
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Air
An eternal sky, the planes of Air are simultaneously the second-most empty elemental realm (only after Void) and the second-most hospitable to life (after only Water). Depending on how deep within the realm you venture, the sky may be an eternal day, twilight, or night sky (in some areas even showing an Astral starscape). The air itself may be luminescent, or suns (believed by some to be a Slip to Radiance or Fire) may roam across the sky, forever out of reach. Microscopic stars may drift through the air, or strange constructs of metal, stone, fire, ice, or pure magic illuminate the skies in other realms. Flying islands the size of entire worlds drift through the planes, storms trillions of miles across roll across the realm, and winds both fair and foul sweep across the skies. Gales strong enough to scour stone to sand, lightning bolts as wide as a continent and so long they take eons to strike their target. Everlasting blizzards, hurricanes traveling at the speed of sound, and debris flung from distant explosions are some of the most visible of dangers, but are no more deadly than the invisible pockets of dead or poisonous air which serve as an eternal threat to those who need to breathe, as vast currents of wind could bring them to an unsuspecting world with almost no warning.
In some areas, gravity may be balanced such that one could fall forever. In others, a passing breeze may blow two mountain-sized islands together in a crash which sounds startlingly like thunder. In some common yet no less bizarre locations, the normal realm rules for gravity seem to be nonexistent, leaving entire islands loosely floating together and with walking rendered impossible.
Air’s Genesis usually triggers at the heart of massive islands, but quickly demolish their creators as endless winds spew forth and leave a cloud of rubble in their wake. This is, naturally, the cause of many of the chains of flying islands orbiting a vast storm which are so common within the plane.
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Void
The prime Planes of Void are perhaps the least-hospitable realm to non-elemental life in all the elemental realms. While Fire may be nigh-unsurvivable for most mortal creatures, those beings which are unaffected by fire, or those who use it for food are able to thrive in it.
The planes of void have none of that. While only the deeper planes actively will attempt to Void matter that enters them, even the shallowest of true planes of void is nothing but a vast, empty irradiated wasteland. Not even ground exists, nor does air, and only a few distant pinpricks of light, courtesy of tiny pockets of Fire and Radiance- only visible because there exists nothing, not even Shadow, to block them- illuminate any of the plane. Even those tiny flecks of fire offer little in the way of true nurturing, as without proper wards the light itself is hostile to most life.
If that were insufficient, Void’s Genesis is actively hostile in a way most other realms are not. In addition to creating more void, all of the matter which triggered the Genesis to begin- albeit a substantially more massive amount than required in other realms, about a world’s worth of substance under extreme circumstances- is pulled into an infinitely small point, both becoming void and continuing to sustain the Genesis. Because the substance still exists, it continues to attract other non-void material and even light and grows all the more ravenous and deadly the entire time. As it does so, it is all but undetectable as it Voids all which approaches too closely. Indeed, the only reason they can be detected at all is by measuring how much they warp space and matter around them, or through the poisoned black light they shine with.
Even then, what few motes of elemental dust that have clumped together are impossibly far apart, making travel within the Void all but impossible. Furthermore, magic often does not function properly in all parts of the Void, and nearly all ley lines are confined entirely to at most a single continent-sized mote of dust, though some very dilute ley lines may loosely connect several such worlds and even the stars they surround.
In some planes of the Void, entire elements are entirely absent, leading to odd, magic-dead planar zones, or places where magic does not behave according to the normal rules of the elemental realms. That any life at all is capable of surviving is a minor miracle in itself, but it is telling that alone of all the elemental realms, it is only in the pockets that anything even remotely resembling biological life is able to exist.
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Radiance
Radiance has a reputation for being pleasant to see or visit, but that is largely due to having larger pockets than most other realms, often the size of multiple worlds. Energy is in abundance within the realm, of course, and simply by existing within the plane anything not composed of Radiance already continuously sheds light. This glowing, radiant appearance is a great boon to all life within the plane, and some of the most fantastic life within the cosmos originates from the realms of Radiance.
It is of course this very fact which ensures that Faerie- so heavily influenced by Radiance- is so abundant with magical and powerful life.
Radiance, while not the most efficient building material, can nonetheless be used as such and takes on the appearance of flawless crystal when condensed. This only serves to further emphasize the realm’s brilliant beauty as cities built of the plane’s primary element are constructed and refract the ambient light into wondrous rainbows.
Simply existing within the realm is enough to be constantly feel energized, and it is this very property which proves so dangerous within the deeper portions of Radiance. Eventually, every atom of non-Radiance material becomes so inundated with energy- and with the nature of Genesis, produces so much energy- that it simply dissolves into pure, undiluted Radiance energy itself. Of course, surviving a stage of this process just below complete material dissolution is likely to leave any who experience it changed for the better, as the flood of energy never truly goes away.
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Shadow
Shadow, as befitting its nature, seems to have almost as much variety in its locales as the Material itself. Indeed, it is not uncommon for an unwary traveler to mistake portions of Shadow itself as being part of the Darkrealms, but while the mortal realm bears many passing resemblances to Shadow, upon closer inspection they are dramatically different. To begin, it can be quite difficult to determine whether or not one is within a pocket while in Shadow, as the elemental darkness which composes the planes fantastically mimic most other substances in existence, sometimes even copying color. Accordingly, vast mountains and rivers, trees and oceans, skies and even fires may all be formed out of shadowstuff with the average observer none the wiser. It is much more common for all such materials to be black or at least oddly shaded, but when within Shadow even normal elemental material takes on such an appearance thanks to the nature of world seeds within this realm.
Genesis within Shadow is trivial to begin, and indeed all non-shadowstuff matter constantly creates it in large quantities: shadows, cast onto the ground or some other surface, linger. This quickly causes anything not constantly exposed to light or otherwise cleansed of the lingering shade to become ever darker as shadows accumulate. Commonly traveled paths within the plane grow in height, as traveling creatures cast shadows upon shadows, building up the ground faster than it can be worn away.
The odd, shifting nature of Shadow makes permanent constructions difficult, but it is trivial to create temporary structures for those who know how to work the plane. Many nomadic denizens use this fact to raise entire temporary cities practically overnight, with architecture to rival even the greatest found in Liadi, only for their castles and walls to vanish as they move to their next destination.
Like all other elemental realms, traveling too deep within Shadow is hazardous to one’s health, but unlike most others, eminently survivable. Traveling too deep within the realm will slowly transform any non-shadowstuff material into shadowstuff, often with no external sign. Living creatures become shadow elementals and carry on the same fronts of civilization common on the shallower levels of the realm. Thus, if not too attached to mortality or standard life, it is perfectly feasible to travel as deep within Shadow as desired.