The Planes of Existence: An Overview

One Universe, three factions: order, neutral, and chaos. Hundreds, perhaps even thousands of planes, each one unique, hosting several smaller realms, each tied and separated from our Universe. And yet, the third greatest mystery of all: for what is a plane of existence? What is it that separates us from the Mortal Plane, yet, at the same time sharing the same physics. Where do we begin to study such a wondrous concept only understandable to gods, this...this sacred physics of sorts?

- Aviinzil, Inur. "The Grand Mysteries of the Universe." 14th Decennial ACPA (Astronomy Cosmology & Physics Association) Conference. 1 July 2017, Janiil Kol'vuriil Auditorium, Takriin'kal'miir University. U'liis, Avernus Domain, Zan'nil kii U'liis.

In this Universe, there exist separate planes of existence. Each plane is unique, and where a single or various pantheons make their homes among the Universe. One plane of existence can host multiple realms, separate and smaller worlds or lands that are a part of the plane. A singular plane or realm can also be where a deity resides. Whatever singular plane or realm where deities reside are often called "divine domains."
Planes of existence are anomalies in the Universe, pocket realities with many of the same physics found in the mortal realm. One of the greatest mysteries, how a divine domain came into existence, is almost unknown. The only ones who know the true origins of a plane are those that created them: the Gods. But, the Gods do not divulge such information.
Each plane and their realms are all connected via The Crossroads.

A Search For Truth

Throughout history, many species have attempted to unravel the mysteries of what exactly a plane of existence is. All have failed, even the Archangels and the Ancients. It is one thing to construct a plane, or realm that already exists, but to create one, to defy reality and create a reality within reality, is perhaps better left alone to gods. However, in modern times there is one species that has been progressing further in this matter than even demons and angels: humans.
Only recently has the Astronomy, Cosmology, & Physics Association (ACPA) officially recognize a new branch of physics: Sacred Physics, a branch of physics dealing with matters that seemingly defy reality, and are only typically understood by deities. To the shock of many, humans are credited with the discovery of this branch, their contributions to the world of astronomy and physics, even surpassing that of the Ancients.
Perhaps in the near future, will demons and devils alongside humans, be able to finally explain just what is a plane. Of course, it is not only Infernus that is studying sacred physics, but also the inhabitants of Paradiso. This has created a race as to which realm will be to explain the true origins of planes.

Significant Realms/Planes

The Mortal Plane

The Mortal Plane is the plane where Earth is located, where humans call home. It is both the 1st and oldest of every plane. The Mortal Plane is a plane of complete neutrality due to the Law of Absolute Neutrality, which is a Primordial decree stating that no single pantheon, nor deity is allowed to interfere within the plane. The Mortal Plane is not considered a divine domain, even though it is rumored that some deities travel it.

The Crossroads

The Crossroads is a central nexus that connects every plane. The Crossroads itself has no ruler and was created by the Primordials and the Elementals. There is complete neutrality in this plane. To keep the plane in complete neutrality, once it was finished, The Fifth implemented an unseen force that is capable of making mana null. This unseen force also has an effect on living beings capable of holding mana; anything living that has mana will suffer from extreme fatigue, eventually becoming comatose if left enough in the Crossroads.

Infernus

Infernus is the second plane to have been created and is also the first and oldest of the divine domains, founded by Zan'nal'takriin. Infernus is a plane belonging to the faction of Chaos and is where demons and devils reside. After the 4th Holy War, humans were allowed passage into the plane, reincarnated as devils. Infernus, contrary to popular belief, is not a blazing inferno. In fact, the plane is identical to Earth.
Infernus is a planetary plane, with 7 continents, each a representation of Zan'nal'takriin's greatest virtues: the 7 Deadly Sins.
Deep within Infernus, locked away for all eternity for their crimes in the 3rd and 4th holy Wars, is the Mesopotamian pantheon. No one except Zan'nal'takriin himself and the other Primordials knows of the location of the pantheon.

Paradiso

Paradiso, also called Heaven by humans in the Mortal Plane, is the third created of the Planes and is the second oldest of the divine domains, founded by Ri'ful'dan. Paradiso belongs to the faction of Order and is where angels reside. Unlike Infernus, humans have been allowed into the plane well before the 4th Holy War, as Ri'ful'dan has deep interests in them.
Paradiso is divided into 3 separate realms known as the 3 Spheres of Paradiso. The 3 Spheres are the Outer Sphere, the Inner Sphere, and Sanctum.

Olympus

Olympus is the plane of existence belonging to the Greco-Roman Pantheons and is the third oldest of the divine domains. Ruling over Olympus is Zeus, one of the first of the Elementals. Olympus is divided into two primary realms: Olympus itself, where Zeus makes his rule, and the Underworld, ruled by Hades, and where the souls of the dead reside.
The Underworld itself is split into several smaller realms. These realms are Underworld Entrance, Tartarus, Elysium, The Fortunate Isles, Asphodel Meadows, and Mourning Fields. Each Realm is completely different from the other.
Because Olympus is many realms, each separate from each other, Olympus as a whole does not belong to a single faction. Each realm can belong to either the factions of Order, Chaos, or Neutrality.

Yggdrasil

Of the planes of existence, Yggdrasil is the most complex. Like Olympus, Yggdrasil is divided into separate, smaller realms. However, the plane of Yggdrasil as a whole is much more complex than that of Olympus, as Yggdrasil is adjacent to the Mortal Pealm; that Yggdrasil is indirectly connected to the Mortal Plane, not needing to venture into the Crossroads to travel between the two.
Yggdrasil is divided into 9 separate realms. The most notable of realms is Asgard, which is connected to the Mortal Realm, which also happens to be Midgard. The other realms are Alfheim, Hel, Jotunheim, Muspelheim, Nidavellier/Svartalfheim, Niflheim, and Vanaheim.
Ruling over all of Yggdrasil is Odin. By his decree, all of Yggdrasil, save for Midgard/The Mortal Plane, all belong to the faction of Neutrality. There are some among the inhabitants of Yggdrasil that heavily disagree with this alignment.

Deadside/The Plane of Agony

Of all the planes, Deadside is where the truly damned go. Deadside is reserved solely for the absolute worst criminals of any pantheon. Inside the plane, there is only torture, both mentally and physically: the souls inside to be faced with agony until the end of time. It is said that the souls here are in so much agony, that the entire plane itself wails. Deadside is the place where darkness and fear, all become a reality.
Deadside is very bleak, and there is no concept such day or night, only a greyness that bathes the plane. Life is also a non-existent concept, the plane completely barren, save for the pools of blood found within.
Deadside has no rulers, it is only overseen by a single pantheon or set of high beings at any given time. Overseeing the plane switches every 500 mortal years. At the current date, Mortal Year 2024, the Loa oversee Deadside.

The Twilight Domain

Considered to be a mythological place to most, The Twilight Domain is a plane that was not created by any deity. This plane remains a mystery to even the 5 Primordials and is even said to predate the Mortal Plane itself. There are only rumors of the place even existing, and any evidence of the truth of its existence is either locked and hidden away, or silenced.

Plane vs. Realm vs. Divine Domain

Although each term can mean separate things in scientific settings, most of the common populations use each interchangeably in everyday life.
In a scientific, or academic setting, it is important to distinguish between the three terms.
A plane is a separate reality from the Mortal Plane, otherwise simply called The Universe or the Primary Reality. Planes of existence are always singular entities, connected, yet detached from the Mortal Plane. For this reason, Planes are often also called Secondary Realities.
Realms are smaller worlds or landmasses found within a singular plane. A singular plane can have multiple realms. A realm can be a separate, detached world from its plane. These are often called tertiary realities and are usually small.
Realms can also be landmasses, attached to the reality of its Plane. These realms are usually large.
Divine domains can often be confusing, as the term can refer to both a plane and a realm. To simplify, a divine domain is where any deity resides. If a plane is only a singular realm or has multiple realms that are landmasses (like Infernus), then the Plane as a whole is considered a divine domain.
Planes that have tertiary realities may not always be a divine domain. If a plane with multiple realms, with each realm having a deity residing in it, it is considered a divine domain as a whole.
If a plane has tertiary realities, but only a select number of realms has a deity residing in it, the plane as a whole is not a divine domain. Rather, the singular realm(s) are considered the divine domain(s) (Yggdrasil is an example of this).
In rare cases, there are planes of existence that are exceptions. There are only a handful of these, all named as domains and not planes. It is unclear as to why this is so, but every one of these exceptions had been named before the Neutral Eon. The Twilight Domain is one such of these bizarre exceptions.