Extract from the preface of ‘Introduction to Cosmic Realmistry and Planar Structure’ by H. Igran.
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The study of Realms, Planes, and Demiplanes, known collectively as Realmistry, is the primary goal of this book. First, some establishment of terms:
A Plane, as used in this work, is any single infinite reality which may be traversed without the use of Slips or Gates (whether artificial or natural). This includes the Plane of Fire, the Base Material Planes, as well as a single layer of Heaven or Hell.
A Demiplane is a single non-infinite reality. They may be small, the size of a pocket (as can be created by Realmistry), as large as a mountain, or larger yet.
Occasionally, a plane (such as the Bifrost) may not yet be proven whether it is infinite or not; such locations are known as Quasiplanes.
A Realm is a collection of planes and demiplanes which all obey similar rules. For example, the Realm of Fire consists of the Prime Fire, Volcanis, the Ashwastes, and more. A single Plane or Demiplane may be a part of multiple Realms (such as Volcanis being part of both the Realm of Fire and the Realm of Earth), or a single Realm (such as the Prime Fire). What determines which Planes are a part of a given Realm is discussed beginning in Chapter 2.
Understand, of course, that the Cosmos is impossibly complex and impossible for any mortal mind to truly comprehend. Even our best models are woefully inadequate to describe the phenomenal beauty and intricacies of the Planes and Demiplanes which make up our reality.
That is not to say that Realmistry is completely intractable, of course. Centuries of work and countless theories have left us with a set of models which can independently aid us in our pursuit of interplanar navigation and exploration. These are, of course, the Standard Model, World Tree, Crossroads, Crystal Spheres, and Glorious Nebula models.
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STANDARD MODEL
The Standard Model is not especially concerned with the actual structure of the cosmos, but instead focused on the various types of realms which. Broadly speaking, the Standard Model divides planes into five categories: Mortal, Elemental, Divine, Primordial, and Liminal.
Mortal realms are assumed to be the most familiar, as they are the realms where mortal life is most commonly found. This includes the Base Material planes, the Faerie realms, the Darkrealms, and in some models the Ethereal realm.
Elemental realms are exactly what the name suggests, those overwhelmingly dominated by a singular element, and includes the realms of Fire, Water, Earth, Air, Void, as well as their overlapping regions.
The Divine realms are those which the majority of Spirits originate from, such as Heaven, Hell, Elysium, Hades, and other realms which create angels, devils, einherjar and other fully sapient Spirits via natural or automatic processes.
Primordial realms include the Outer Chaos, Maelstrom, and Green Lands, those realms which embody a natural process or are themselves said natural process (as is the case of the Morningstar). While they can create Spirits on occasion, they are rarely sapient from inception and must parasitize mortal souls (like demons) or be given the gift of a mind by a deity to truly gain awareness.
Lastly, Liminal realms include the Astral Sea, the Ethereal (when not classified as a Mortal realm), and other connective planes such as the Winding Path and Yggdrasil.
The Standard Model is generally useful as a shorthand, as realms within each category usually obey similar laws of reality (the Elemental realms being the most obvious example of this) and with the obvious exception of Liminal realms, it is typically substantially easier to travel within a single category’s realms than between categories.
This model has obvious flaws, of course. Some versions classify the planes of Light and Dark as Elemental, others as Primordial. Most notably of course, it does absolutely nothing for actually helping with interplanar travel, making it of middling use to aspiring planar manipulators and travelers.
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CROSSROADS
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The Crossroads may be broadly considered as a variation of the Standard Model actually useful for Realmistry calculations. Instead of directly categorizing different Realms, it assigns a set of values to each plane in existence, based on a number of measurable factors, which primarily aid in determining approximate distances between planes. While variations exist, the most common values utilized in calculations are Chaos, Order, Radiance, Life, Elemental, Divine, Power, and Mana.
For more information on each of these measures, see Chapter 2.
When used in conjunction with the Crystal Spheres, the Crossroads model is the primary method of calculation used in Planar Orreries assuming Slips are ignored. The Crossroads are also quite useful when attempting to estimate the distance between planes, and the tools it provides are exceptionally useful when attempting to divine from nothing your location in the cosmos.
Unfortunately, the Crossroads are very inexact. In addition to not accounting for temporary or unusual planar conditions (making it useless for planar alignment work), treat all numbers you learn from your calculations as estimations, useful when traversing the Bifrost but never to be used when actually constructing a Gate or attempting to Planeshift.
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WORLD TREE
The first model which actually provides proper structure to reality is the World Tree model. As per the name, it takes the form of a colossal tree stretching as tall as the cosmos is wide. Every leaf, root, and flake of bark is a distinct plane, all connected as trees are.
The roots of the World Tree stretch throughout the Elemental realms, which serve as the foundation to the rest of reality. The trunk of the tree is the Base Material, with low-lying branches representing the Darkrealms and Arcadia. Eventually, it branches into the grand crown of the Divine realms and the vast complexity to be found there.
In contrast to the Crossroads, the World Tree model is highly accurate for local Realmistry, regardless of where in the cosmos you are, so long as you are utilizing exclusively natural pathways, such as actual world trees, cosmic rivers, or natural planar gates to travel. Its branches do an excellent job of mapping out which planes and realms are adjacent to one another.
It is still not a perfect model, naturally. As a simple example, given that Light and Dark are considered part of the Elemental realms, which means that it would map Arcadia and the Darkrealms as liminal in respect to the Elemental realms instead of to the Divine ones. Furthermore, it does little to account for the Primordial and Liminal realms. The former, it suggests as being beyond the tree’s canopy and the latter simply does not fit within the model at all. Accordingly, it offers no explanation of Slips (which it considers merely another form of Liminal plane despite being quite incorrect) and does absolutely nothing to account for planar drift, making it utterly useless for calculating planar alignments.
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CRYSTAL SPHERES
In strict contrast to the World Tree are the Crystal Spheres, which, inspired by the view of planes from the Astral Sea itself, posit each individual plane as one of the eponymous Crystal Spheres floating within the Astral Sea, which is itself is in turn a series of crystal spheres resting within the Chaos Beyond.
Realms are groups of planes which naturally cleave together tightly, staying in formation even as they drift and move in accordance with Astral forces, and liminal realms are akin to currents within the Astral or the Astral itself.
This is the primary model used to determine proximity between planes for artificial travel, as well as a method for determining the effects of non-neighbor realms on one another. It is sometimes seen as a refinement upon the Crossroads, as it utilizes some of the same foundational principles for proximity calculation, but differs in a number of important ways- most notably in that it focuses on planes rather than realms.
It is, of course, hideously difficult to do any appreciable work with for all that it is substantially more accurate at predictive capabilities than any other alternatives. Furthermore, it offers no proper explanation for Slips and cannot account for them.
GLORIOUS CLOUDS
Perhaps the most technically correct model is the Glorious Clouds, as it and its predecessors are the only which actually are able to account for Slips. It posits that, with a significant amount of simplification for the sake of clarity, all realms are something like a diffuse cloud drifting through the Outer Chaos, intermingling and being disrupted by various forces. Adjacent realms are those which have significant overlap in area, while Slips are the result of a tiny amount of a plane being separated from the greater whole and drifting into the area of a plane they do not normally interact with.
Obviously, the primary use of the Glorious Clouds is to account for Slip behavior and interaction. No other model has anything close to a proper explanation as to what causes nor how Slips manage to connect even the most disparate of planes in such a miniscule area.
While very conceptually useful, the Glorious Clouds have very little actionable which can be done with the model. They are an important factor to consider with truly delicate Realmistry rituals, details for which are laid out in Chapter 26, but due to the odd nature of planes within the model, for nearly all situations which might utilize it, either the Crystal Spheres or Crossroads will function just as well.
Choosing the proper model for your application is quite important, after all. Accordingly, as we move to Chapter 1 we will begin with an in-depth look at rituals which must utilize one model or another, and how to convert between models for truly sensitive applications.