The planes are not unlike a layer cake with a surprise in the core.
The “plate” on which the cake rests is the Primordial. It’s said that the building blocks of all creation can be found here. It’s the source of power for Sorcerers with an affinity for wild magic. It’s the home to elementals and mind bending displays of physics. Scholars posit that many of the more unusual features in the material plane are the direct result of fissures in our reality that connect to the Primordial. Few who have visited this plane have survived.
The most bottom layer of the cake is the Infernal Coils. Nine rings coil into a a swirling central maelstrom known as the Abyss. The Abyss is the most direct (and dangerous) route to the Primordial. Devils rule the rings and tend toward lawful evil alignment while Demons reign over the Abyss with and are chaotic evil in nature. In either case those judged to be evil after death live out their afterlife here.
Purgatorium is the middle layer of the cake. It envelops the Material Plane and The Fae Realms (the core of the cake). It is a shadowy realm where neutral beings and ghosts who don’t wish to pass on can be found. Only 3 Gods make Purgatorium their home -
information smudged out. Their judgment is required for most souls to be sorted into the Astral plane or the Nine Hells and Abyss.
It may seem odd to combine discussion of these two planes under one heading but, as the “core” of the universal cake they are where the vast majority of creatures live. Each plane exists slightly out of phase with the other but there are many points of overlap if one knows where, and more importantly when, to look. The study of the Zodiacs of Yggdra is largely responsible for figuring out the mysteries of traveling between the two planes. The portals that connect the Thule Consortium rely on warping the weave between these planes and are remarkable in their reliability.
Some scholars in the guild believe there is a 3rd plane that is even more out of phase - the Draconic Plane. They surmise that Avalon isn’t so much a land that exists on our world (or on another known plane) but is it’s own plane of existence whose portal opens rarely. Any dragon ruins that house information on the constellations at the time of the dissapearance should be considered of singular importance.
At the top layer is the astral plane and is home to the angels, many of the benevolent gods, unicorns, blink dogs, and other creatures of celestial origin. For the sake of simplicity, and at the risk of oversimplifying, this is the plane of good aligned creatures and where those live a good life are allowed to spend their afterlife.
The Ethereal Plane is not so much a plane as a membrane located between the core planes and Purgatorium. Souls pass through it on their way to the afterlife, some creatures use it when they cast the Blink spell or turn invisible. The only permanent creatures you will find here though are ghosts and similar undead who resist moving on.
It’s possible other planes exist slightly out of phase (as the Material and Fae realms do). If you find information on such places, please add it to the archives.