The many different planes are all inhabited by other beings. If the current theory holds, then every person on Wyrlde has a match in a different dimension, but those beings aren’t always people, like us, even if they are, in fact, people who have once existed and passed on.
All the principal residents of the assorted planes are ultimately descended from the events before the God’s War. We call them Denizens. They are the literal children and grandchildren of Gods. They were often cast out by Belial when he ruled all, and more than a few are his issue. Later, some of them were turned to for help during the God’s War (notably the Fairywilde).
The Seven Planes each have their own structure, their own way of being. We call each of the Dimensions within each plane a particular thing, often from concepts handed down through the eons by the Ancient Ones. Each of them can be startlingly different in and of themselves. The Dimensions are all separated from each other by a Veil, and the veils are separated by the Pale, and so they connect and sit beside each other, each occupying the exact same space but still set apart by the Veils. This is readily shown by the status of the Dimensions of the Mortality: Ephemeral, Fairywilde, Astral, and Ethereal. Each plane reflects these same four things back throughout, and so some correlation can be made among (such as the Heaven, Fairywilde, Hell, Euthania, Limbo, Luck, and Pandemonium correlation).
It is key to note that each Plane has people who are beholden to it, and that at least one of the Inner Planes in each of the Outer Planes is essentially a complete mirror of Wyrlde. These complete mirrors are called The Seven Mortal Realms, for they are the central aspect of each of the planes in the same way that Mortality is the center of the Mortal Plane.
Each Domain reflects some aspect of the Plane of Mortality, both as a whole and in its parts. For example, Wyrlde is a part of the Ephemeral Dimension, and is reflected by Yrthe, Kismet, and Shades in different ways.
Likewise, Heaven, Hell, The Dreamscape and Nightmare are all reflections of the Fairywilde, as well, and this follows throughout.
There are obvious oppositions in play Celestial vs Infernal, Radiant vs Necrotic, Shadow vs Nether. There are also less obvious oppositions: Shadow vs Necrotic, Radiant vs Nether, Radiant vs Shadow, Nether vs Necrotic.
There are conflicts of a tripartite nature beyond those: Celestial/Nether/Necrotic, Infernal/Shadow/Radiant. The relationships and hostilities are constant and involved and become issues that the Powers must deal with when engaging the service of Denizens.
Sometimes it helps to have it laid out simply, for understanding, but it always essential that one remain aware that all of them occupy the same are, just “out of phase” or “at a different frequency” from one another, like a bunch of marbles all in the same space at the same time.
This shows the relationships between the assorted dimensions, and thereby the relationships between beings like the miasmata, Fae, Angels, Devils, Shadows and Nightmares.
In terms of cooperative natures, Celestial/Radiant/Shadow and Infernal/Necrotic/Nether are the best known.
One of the Dimensions for each Plane is always a Mortal Realm, and Mortal realms are echoes, reflections, shadows, or some form of variant of each other. It is a difficult explanation to give more accurately, since the planet could be completely different, with different names for everything and may not even have magic (as it is said is true of Shades). And yet, they are all part of the same cycle and ultimately are the afterlives we often speak of.
Sometimes people from them travel to us, while people from here have sometimes gone to those places, excepting Shades, from which no one has ever returned. In some of them, the Gods live and exist under different names, perhaps with the Old Ones being more powerful or the Powers behaving completely differently.
Each Mortal Realm has an additional challenge in that just as there is the Fairywilde and the Astral and the Ethereal here, they have their own reflections and interact in similar ways with that realm.
Of the Mortal Realms, the two that are closest in nature to Wyrlde are Yrthe and The Bleak.
Yrthe is similar in many ways, though they call Professions there “classes” and they have very different ways of understanding things that often create confusion when they cross over here, or we cross over there.
The Bleak is essentially Wyrlde but without magic as we understand it. There it seems a lot of folks have mental mind powers similar to our psychic ones.
Shades, as noted before, has no magic, no mind powers, noting. It is a strange place that reacts very badly to those who do appear if they are seen doing things using magic to arrive. The downside to it is that once beyond the gate or path of arrival, there is no magic, and so there is no chance of return. People do not regain Mana there, and it bleeds out of them slowly, eventually leaving them with little more than dregs.
The assorted Denizens of the Planes have a somewhat complex relationship to the others. In most cases, this relationship can be described in a single term, the ambiguity of each fully embracing the possible ways in which these beings engage with one another. It should be noted that all the major beings are ultimately the children and grandchildren of the Powers That Be, and that their relationships with those Powers are distinct and separate from how they relate to each other.
In all cases, the Denizens of the Planes can be counted on or found serving assorted Powers in varying capacities, most often acting as servants, messengers, and general support staff for the Powers. Sometimes they are given tasks to handle within the Material such as guarding a potential Ikon or supporting someone of interest to a particular Power.
A single Power may have a Devil, Demon, Wraith, Angel, Valkyrie, Ghast, Ghoul, and so forth all serving them at the same time in the same place. This does not mean they get along, however. The following charts show the most common general reactions of the assorted Denizens to one another.