The Creation

In the beginning, there was simply the Lord Primordial. It created the multiverse and all within. As such, there was no sense of time, just nothing. Mist and shadow - yet not in the way we perceive them- for there was no light, and there was no dark. The Lord Primordial opened up the boundaries of each universe to let in the Elements and shape life. However, these raw elements were unwieldy, and gave way to utter chaos, and so It encapsulated this Chaos of Elements. From the Chaos, It formed four children, within them each the purest major elements of Fire, Earth, Air and Water. Together they bound the rest of the Chaos within a sea of light and energy and called it the Planar Astaria- the Astral Plane, created from the Lord Primordial's own incorporeal form. From the Chaos Primordial, They formed the first true Plane - the Planar Celestria, the Celestial Plane, the Ancient Grove.
At the crux of it all, the Lord Primordial fashioned for Itself a plane of possible impossibilities. The Primordial of the Earth lent Its body to become the infinite expanse of land atop which the Lord Primordial's mountain would sit. The Primordial of the Air lent Its intangible form and immeasurable power to suppress magic throughout the plane, preventing accidental discovery by mortals. The Primordials of Water and Fire lent Their powers and bodies to combine with the elements which existed there. The Water combined with dust from The Earth to create forms which The Fire breathed life into. The Lord Primordial Itself formed an infinite mountain atop which would sit Its creation of mighty defenders, guarding the gateways and pathways between planes. Their work complete, the Lord Primordial's first children went into a deep slumber, to await a time when they would be needed once more.
Within the infinite vale of the Ancient Grove, the first 'beings' were formed. Much like the Lord Primordial, many of them were incorporeal, with ever-changing forms. The first of these beings was Daraphiel, who chose the form of what He called the ‘snake’, fashioning for Himself wings, that He might soar over the land below. With the Lord Primordial's permission, He bound himself to the weave of what the Primordials and the Gods called "Time", that He might become it's sentinel, stretching His body to the very beginning- and He gave meaning to the concept. As these Gods experimented with their power for millennia, They created a world below with which they could experiment. It was almost universally agreed, at this point, that eight among Them should be elected to form a 'council' and thus the Divine Council was formed. The Council, through Daraphiel's divine influence, created the true meaning of time, triplet moons- Kaas, Dag and Pan - and shaped a part of the Chaos Primordial into what mortals would call the Sun. Thus began the First Age of the First Era of Erroth.
Gradually, the various Gods began to spread Their influence slowly across the Mortal Realm, across the many millions of years it took for life and civilisations to properly develop. The Mortal Plane was home to simple life, at first there were humans, and simple creatures, but the powers which had been infused into the Creations and which coursed through the land of Erroth were proving too chaotic, twisting the other God's creations- giving birth to Orcs, monsters, and various other dangerous beings. Bahamut, working with Kafziel the Divine Protector, created the metallic dragons to serve and protect the land, but the chaos of the elements began to twist them too, giving birth to Bahamut's sister Tiamat and Her forces of chromatic dragons.
The Fey Plane was created at the same time by the Divine Council as a physical manifestation of the raw power that was harnessed by the Gods for creation and was being slowly discovered by the inquisitive wizards on the Mortal Planes. The manifest of such power also gave birth to new flora, fauna and creatures. The Elves were created when the Gods placed these Humans who had tapped into this power within the Fey Plane to study the effect of prolonged exposure to such raw power. This plane overlaps Erroth as an alternate plane occupying the same cosmological space. The landscape mirrors the Material Plane, but in a more spectacular fashion.
Some among the Gods didn't agree. Skaomune, who would become the Lord of Darkness, disagreed with many of the Gods in the Ancient Grove. He saw His later placement on a ninth seat of the Council as an insult - as if it were inferring He was a lesser God. When He rose up and spoke against Them, the Divine Council chastised Him, casting Him out of the Council and passing His seat to the Dragon God Bahamut. So He began to work in secret, gathering a following of other Gods who felt that They, too, deserved a seat on the Council. He had watched the actions of mortals and immortals alike for the first millennia, using His dark power to twist the negative energy released as a by-product of the God's creation magic, forming it into a dimension of black, grey and white, where most colour is leeched from everything. Much like the Fey Plane, the Shadow Plane also overlaps the Material Plane- landmarks are recognisable, but twisted and warped. Skaomune and various other Gods who were unhappy with the Divine Council embraced the Chaos Primordial, bending it to Their will and pitting the twisted creations against those of the Divine Council.
Their plan to overthrow the Council was discovered however - Daraphiel, who knew all that is, was, and would come to be, knew that action must be taken. After debate with the Lord Primordial, a being with whom contact was rare for Its presence and location was known only to the Master of Time, They agreed that these betrayers who sought to do evil must be separated from the Gods who sought to do good. So the betrayers were brought before the Divine Council. Their sentence was to be separated from association with the Divine Council.
As an act of mercy, wishing not to cause strife amongst the Gods, Maroth tore a piece of the Ancient Grove loose and cast it below, where it became the Infernal Dominion, over which the Fallen Lords could rule. Their lust for power, however, led to a fight which lasted centuries - spawning demons and devils to do Their bidding, and tearing the Infernal Dominion asunder. The Fallen Lords saw Their mistakes and came to an agreement. Though They liked each other not, through collaboration, Their power matched and rivalled that of the Divine Council, and so Their own Council was formed. The Council of the Fallen Lords, over which Skaomune would rule, and seven others would balance.
Thus began the Second Age, and thus the Councils were truly formed - eight on each, though the Divine Council included a ninth seat upon which Gods who were not initial members of the Council could sit, providing legitimacy, balance, and an outside voice to raise concerns over which the Divine Council may have passed.
Skaomune waited, biding his time within the Infernal Dominion, slowly corrupting other Gods to his cause, such as Fendel, who had begun to secretly experiment with the powers of the Shadow Plane, and it was through this that Skaomune began to warp his mind. This corruption spanned the first million years of the Second Age of the First Era as Skaomune began to build an army. First, he struck deals with the Devils and Demons, vile creatures created by his own experimentation, twisted and made stronger by the Chaos Primordial before The Sealing. These two factions, however began to resent each other, each believing themselves to be the better.
The Lord Primordial watched these developments from Its resting place at the crux of the universe, and was unhappy with the chaos that Its experiments with the raw powers were causing. In an event which would become known as The Sealing, It separated the pure raw elemental powers from the material plane, using them to form various barriers which would harness and control the Chaos Primordial. It thrust these defences across the Planar Astaria, where they could prevent the overly inquisitive from accidentally tapping into these powers - if one were to even know where to look, one would simply see but a reflection of the Material Plane and of their own self looking, reflecting infinitely and driving the looker to insanity. With the dawn of this Third Age, memories of the existence of other planes began to fade from the minds of mortals.
After millennia of corruption, underhanded deals, and minor attacks and invasions of planes Celestial and Mortal alike spanning the prior ages, the Fallen Lords declared all-out war on the Divine Council above. Thus began the Second Era, and thus do the Lord Primordial and Its forces watch from their seats at the crux of the universe, awaiting the outcome of this ill-fated war...

Summary

In the beginning there was nothing but the Lord Primordial, formless, without classification. It created the Gods themselves, who decreed the creation of a Divine Council to watch over the planes and the creations. The Lord Primordial was unhappy, however, for the raw power of the Chaos Primordial which had been infused into Its creations was corrupting the creations of the Gods, and some of the Gods themselves. In an event known as The Sealing, It separated these powers to prevent their abuse and formed them into planes which mirrored the Material Plane, but were sealed away to prevent their access. It is said that anyone attempting to look for these planes sees only a reflection of themselves looking. The Lord Primordial and Its four elemental children created a plane above all others, to sit in the centre and serve as a bastion of planar protection.
Whilst this occurred, all was not as it seemed with the Gods. Skaomune was angry that His 9th seat on the council seemed to imply He was a lesser God and began to scheme. He created a plane of negative energy through which He and those He would convince to join him could draw power and create, unbeknownst to the Divine Council. Milennia after The Sealing, with the Gods having just recovered from the separation and redistribution of Their powers, Skaomune and the other Fallen Lords, who had been banished by the Council to the Infernal Dominion within the Shattered Realms declared all out war on the Gods of the Celestial Plane.
The Gods have been at war since.

Historical Basis

It is clear that The Creation occured, for life must have originated somewhere.
The notion of The Sealing is apparent, for there many have tried and failed to discern whether planes other than the Material (Erroth), The Æther (The Fey Plane), and The Obsinarius (The Plane of Shadow) exist. There are often crazed reports of individuals claiming to have looked through crystal balls, mirrors and even windows to see other worlds which mirror Erroth in many respects, but are different in others.
The vague notion that other planes may exist is known, but frowned upon as a taboo subject. The ideas of fairies and pixies from alternate planes; of Devils making contracts of power in exchange for mortal souls; of crazed, chaotic Demons scheming and plotting to bring chaos and ruin to everything; of strange, alien creatures, who's aims and abilities are unfathomable are nought but fairy tales, bedtime stories told to motivate or warn children.
The War is known widely and is feared. It has given birth to three factions of mortals: one blames the Gods and Their Wars for the destruction caused, seeking to bring the Gods to their knees to answer for their crimes; one blames the hubris of Mortals and their disbelief for weakening the Gods, allowing the War to begin; and one blames the forces of evil and the forces of chaos, for law and order are needed to return the realms to peace.
Whatever the case, the fallout of the War is massive, and the involvement of such large powers has prompted the mortals to develop methods of defence against collateral damage in most cases.

Spread

The historical basis of this mythos is widespread knowledge. The extensive knowledge of the mythos of creation is often regarded as the sacred knowledge of clerics and of the devout, making it less common.
Knowledge of The Sealing is usually considered taboo and thus apocryphal, as is extensive knowledge or study of the notions of planes and planar travel. Despite this, many clerics or devout followers of the Gods may learn the vague notions of this event to demonstrate the power the Gods once wielded and show that even the Gods are not without their faults - all must answer to higher powers.

In Literature

There are many books detailing The Creation mythos, with very few going into detail about the idea of planes - referring to these other creations as no more than stars - for example, the Stars of Law, Chaos and Fire, which are part of constellations, the names of which are referred to as the "Lords" of these focal stars.
Many cultures ban such books, deeming these works sacrilegious and apocryphal. Seeking such knowledge in these cultures is a taboo of the highest order and so these works are rarely found.

In Art

Paintings, tapestries and the like depict sections of the mythos such as a formless being - often represented as a divine light - creating the universe, the Gods in their believed forms - giant winged snakes, massive birds, humanoids and the like - creating life itself, and various other aspects of the Creation mythos.