From ‘Introduction to the Planes,’ by T. Ketzvih:
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THE LIMINAL REALMS
The Liminal realms are an unusual grouping. Not bound together by planar rules so much as the other realm categories, this is instead the primary name for those planes and realms whose primary feature is that they connect several other planes and realms together. Creatures native to Liminal realms have little in common with one another when they do exist, and may take after any other type of creature within the cosmos.
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Astral Sea
The Astral Sea is the only true Liminal realm (others being merely planes or demiplanes), and is difficult to properly describe. Its influence creates the Ethereal in the mortal realms, and is in some ways the strangest of all realms as it weaves across the entire cosmos, bordering every plane.
In the shallow Astral planes, Thought and Will have tangible effects on their surroundings. People begin to show faint reflections of their innermost drive within their eyes, and moving is simply a matter of desiring as much. As one ventures deeper, their form changes to reflect their true being, and thoughts are just as tangible, if not more, than actual matter. Eventually, it is said, there is nothing but thought at the deepest reaches of the Astral Sea, ideas echoing across the planes and shaping reality.
Traveling within the Astral is mostly a matter of imagining where one wishes to go, and the plane will naturally draw you to that location. As the Astral has locations truly dedicated to every thought in existence, travelers must take care to school their thoughts lest they find themselves drawn towards the Isles of Id.
This process is naturally faster the deeper within the Astral one is, but the deeper one goes the higher the risk of fading into nothing but one of countless thought-echoes littering the realm, serving as the final memorial or remnant of ideas and beings so notable, potent, or close that reality continues to remember their existence millenia after the original died or moved on, a single eternal thought, echoing across the planes.
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Akeron
Akeron, also known as the River Akeron or the Akeron Sea, is the largest Cosmic River and the only one proven to be a true Plane. Its headwaters begin in the wake of Felisel the celestial mountain, before winding through the rest of the divine and primordial realms, as well as a good amount of the mortal realms, before joining the endless oceans in the realm of water. Yet its headwaters are also within the realm of Water, and it sweeps through the mortal realms and terminates into the divine realms, flowing both ways simultaneously. Yet others say it originates deep within Faerie, while others say it originates from the mythical Evergarden at the center of creation. Some say that it is a narrow stream with haunting banks all too close to the river’s surface, others that it is a wide river with welcoming, grassy banks, and yet other reports still have it as a vast ocean only rivaled in size by Water itself.
Regardless of the specifics of the plane itself, with a skilled navigator Akeron is easily the fastest natural method to traverse the planes, as distance is nigh-irrelevant. It takes but hours, days on the outset, to reach anywhere Akeron touches (which is to say, nearly anywhere in the cosmos which possesses liquid water).
The unwary traveler must still be cautious, as the plane is not without its hazards. Without a skilled navigator, it is remarkably easy to find one’s self hopelessly lost within winding riverbanks, whose currents rarely match the logic adhered to within the mortal realms. Or, alternatively, be lost to one of the endless dangers that lurk within, drawn from every drop of water within the cosmos.
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Bifrost
The Bifrost is the most prominent quasiplane (that is, a plane whose status as demiplane or proper plane is inconclusive) in modern Realmistry, as while it certainly seems finite in size, no one has ever managed to conclusively prove whether or not it is.
The simplest description of the Bifrost is that it is a radiant, prismatic path or bridge. It shimmers with incredible beauty to all who see it, regardless of what senses they use to study it. Magically, it is a glorious tapestry of every form of mana (as known by each end of the Bifrost). Visually, it is a radiant weaving of every color the observer is capable of seeing. Auditorily, it constantly emits music pleasing to the ear of the listener and of a manner reminiscent of both the start and end of the Bifrost (what is pleasing to the ear, and thus what listeners hear, may vary within the same traveling party). More exotic senses likewise report that the Bifrost has a highly pleasing medley of sensations, and while seemingly home to impossibly complex patterns and shapes, the exact manifestation of such constantly changes and scintillates.
A case of literary theft: this tale is not rightfully on Amazon; if you see it, report the violation.
The Bifrost is unique among the liminal planes in that it always takes you precisely where you wish to go. Or more accurately, it takes all individuals in a group to wherever they wish to go. Upon entering the Bifrost, if no members of a group has a very clear idea of where they wish to go within their mind, they will simply be returned to wherever they left from. If at least one member of a group does have a clear destination in mind, the Bifrost will unerringly take them to that location. In some instances, the Bifrost takes the form of an actual bridge which must be walked along, while others describe it as more of a current or general pull and thus require no action to move.
Regardless, while accuracy is never an issue with the Bifrost, that is not to say it is without its dangers. Should multiple members of a group all have different desires as to where to travel, only the one with the clearest picture (or strongest mind, some say) will win out and pull the others alongside them. Furthermore, it is impossible to truly predict how long a trip through the Bifrost will take. Divinations and calculations can determine an estimate on travel time, but given it is impossible to exit the Bifrost to any location but the end of the plane, the cost of a misstep can be high. On the outside, travel can take centuries or even longer in some situations, ancient corpses materializing in a location where their living selves once hoped to arrive.
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Winding Path
The Winding Path is a solitary, but true, plane. It manages to lead everywhere on every plane… eventually. It takes the form of a path of some form. It might be paved with cobblestones or simply be a well-worn track in the dirt, may appear ancient or brand-new, may be treacherous and hazardous, but it is always the perfect size for groups traveling it. It is trivial to leave the path- simply stepping off of it, or flying too far above it will drop the former travelers in the corresponding point in the multiverse. Rejoining the Path via magic at that point would in theory return you to the same spot on the Path, but even minute changes in location may correspond to vastly different places on the Path. Groups entering or leaving simultaneously are usually able to stay together if they are careful, but when a half-step can make the difference between the welcoming fields of Elysium and the eternal blackness of Void there is always the risk of mishap.
It is not uninhabited, however. While there of course exist many residents upon its eternal path who have found themselves upon the Path and do not wish to or are unable to leave, so too do the mysterious ‘Riders’ call this plane home. Reports vary on what the Riders are, but from those who see spectral steeds mounted by an elven warrior to those who simply see a faint whisp of light carried along by an unusual breeze, all agree that they ought to be called ‘Riders.’ Where Riders come from is just as great a mystery as their true appearance, if an answer to such questions even truly exist.
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Yggdrasil
Yggdrasil is the greatest world tree in the entire cosmos, the only one to be a true plane, and often said to be the absolute first, of which all others are offshoots or children. Like all World Trees, it is one of the stablest natural methods to traverse the planes. Unlike other World Trees, all known planes are located within the crown of the tree, which continuously grows. It does not reach every point on every plane, but where it does reach it is relatively trivial to embark or disembark, and continues to grow at all times.
What is located at the bottom of Yggdrasil is unknown. Some believe that its base and roots are deep within the Outer Chaos, others believe it has no roots and simply extends forever, and some believe that it is but one offshoot of a much, much larger World Tree, connecting other cosmos beyond any being’s imagination. Regardless of the truth, no expeditions sent forth have ever returned with a report beyond describing its trunk, and all of the demiplanes tucked beneath its bark, the strange and dangerous monsters which make their home there, and the paradoxical features which can be found along the First Tree.
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The Darkness Between
The Darkness Between is perhaps the most dangerous of all the liminal planes. It is an eternal, inky black void where all sound, all temperature, and all magic is simply faded away into nothingness. The only way to survive is to obey the strange and often paradoxical-seeming instructions all who enter the plane inherently know yet vehemently disagree upon what said instructions are. It is believed that failure to do so will result in instant death, but this has never been confirmed as none who have disobeyed the instructions the plane tells them have ever emerged.
Some suggest that it is in truth an elemental realm between Shadow and Void, while others argue that with no elemental life to speak of and a lack of any discernible elemental genesis in the wake of normal matter passing through indicates that it is something else entirely. Regardless, The Darkness Between is easily accessed by both of the aforementioned realms and can be used to travel to any other place in the cosmos which has true darkness. This method of travel is strongly reminiscent of using the darkrealms to traverse the mortal realms, but stretches across all of reality.
The actual experience of traversing utilizing the Darkness vary from individual to individual. Some simply find themselves at their destination in an instant, whereas others must walk for hours, days, even years before arriving. This has no bearing upon the actual distance traveled, and even the same journey involving the same individual may result in different experiences.
Even following the plane’s instructions and instincts perfectly is no assurance of safety. Those who spend too long exposed to The Darkness Between feel their instincts develop into whispers, then outright voices. In many circumstances, the whispers and voices may be sufficient to drive the listener mad. Those who later recover often speak of beings of the dark, and who are the dark itself, in descriptions not that dissimilar from the sorts of creatures occasionally described when deep in any realm.
This has led some scholars to speculate that The Darkness Between may be several planes tangled together, and that those who get lost within the plane in truth stumble into a layer of the plane incompatible with mortal sanity, with some of those lost forever wander into a layer incompatible with life or even existence.