Of Magic & Gods

"In a time before time, the world was not yet formed and all that existed was power and the forces that controlled that power. Now, the people of Khorvaire would call this force God or, maybe, gods but then there was no need for names. The force was and that was all that mattered. In time, that force became aware and in its awareness it began to shape things. Simple things at first. Fire. Water. Earth. Dark. Light. As time passed, however, the force of creation began to imagine more. In imagining, it directed more power and shaped a full world.
The planes took shape and existence as it is now known came to be.
This act, however, expended the force that had directed it. Imagination, it seemed, was something difficult for it...despite it being simple for the results of its creation. The power it had wielded was magic shaped through the power of imagination and now mortals are able to wield their own imagination to call on that primal power of magic as well...to varying extents." - excerpt from 'Formative Energies' by Ukalis of Thrane (Chapter 1 - On The Arcane Origin of the Cosmos, Section 3), year 431
The above is the prevailing view of the origin of all existence as accepted by general magical practitioners and intellectuals of Khorvaire (though competing theories also exist including among the elves). Magic is the original creative-power that is able to shape reality itself and through will & imagination this power can be shaped. Wielding that power, however, is no simple task as mortals are far from the source of magic and, therefore, are less capable of utilizing it. For some, this power can be learned through rigorous study of formula that primes the mind to tap into magic through rote memorization. For others, the power comes naturally as breathing. Others still gain the ability through faith in an outward force of incredible introspection and harmony of body & mind. Whatever the method, magic is a tangible, powerful energy that infuses the entirety of existence. Indeed, this power is even found in inanimate objects with Saether being a primary example. Alchemy itself is the study of releasing magic from combinations of materials, after all.
Any discussion of the origin of creation and magic, however, also has to touch upon the other planes. "Purer" in concept, the other planes are often less diverse than the Prime Material but no less dangerous, having been created just before the Prime. Whether reflecting an element or some less tangible concept, these planes also share many similarities with the Prime with some considering their entire existence to be a sort of prototype of the Prime or even the initial left-over ingredients from the creation of the Prime. Naturally, denizens of those planes capable of thought generally considering both of those notions to be insulting.
Nowhere in existence is the proof of the above origin of the universe more evident than in the planes of the Celestials and Fiends. Said to have been birthed from the emotions of the creation force that created the planes, the Celestials represent hope, love and other positive qualities while the Fiends represent the exact opposite. The Fiends are, in fact, so warped with hatred and evil that they war with each other, the Devils & Demons in an eternal conflict. While the Celestials act to preserve the sanctity of reality, Devils seek to enslave & rule it while Demons seek to destroy all things.
With the Celestials, Devils & Demons roles so very clear it heavily bolsters the claims made about the origin of existence...
However, the third fiends, the Yugoloths, claim that they are older than any of their planar brethren and are, in fact, older than the force that created existence as it is known...