I quote from Fiendish Codex I (pg. 5): "Most importantly, I haved learned that the Abyss is the darkling chaos from which all things come. This genesis has nothing to do with good or evil - just pure, churning chaos, in its simplest form." Say WHAT??? And from Fiendish Codex II (pg. 4): "In the beginning - and even before - chaos was all that existed. Out of it came demons - the living manifestations of chaos." WAIT a cotton pickin' minute - the Abyss ain't evil? And shouldn't LIMBO be the primary chaos plane? It's Limbo, after all, that is neutral in reguards to good and evil... NOT the Abyss!!! What where the writers of these two books SMOKING? I thought the SLAAD race was the living manifestation of PURE chaos (well, o.k., so some Sladd lean towards evil - but STILL!) And isn't the bit about the Eladrin race attacking the Abyss a direct steal from one of Green Ronin's books? It sounds AWFULLY familiar... and the Queen of Chaos was originally a native of LIMBO, according to the boxed set "The Rod of Seven Parts"; she only took up residence in the Abyss after turning to evil and recruiting Miska the Wolf-Spider as her general/mate... she ain't no Obyrlith, no matter what it may say in that book!
The Abyss as the origin of chaos/everything?!?
Maybe she was the first obyrith, forming her race as she fell to evil.
Both of those quotes come from the italicized text at the beginning of each book which is "flavor text" and not necessarily meant to be taken as canon or even truth for that matter. I miss the days when they would put that stuff in quotes and attribute it to some barmy local. (3E FR material needs more Volo quotes!)
The first chapter of FC I is called "Demonic Lore" and any lore propagated by fiends is suspect. The fiends don't feel that "evil" is in any way "wrong". To them it is a meaningless title for a superior outlook on existence (that outlook being supreme selfishness and power mongering).
To a demon the Abyss represents chaos in its most pure form and the origin of all it knows or cares about. Limbo is just a weak reflection of true chaos and someday it will be consumed by the Abyss just like everything else in the multiverse.
-420
Fiendish Codex I, was written by the same writers that did Green Ronin's book on Demons. It was also written by the same writers that wrote the Demonicom articles in Dragon and the various Adventure Path series of articles in Dungeon.
They've expanded on and changed a lot of things with that war from Rod of Seven Parts. Other writers eventually added that some of the Elemental Princes of Evil fought for Law, and some of the Elemental Princes of Good fought for Chaos, later on in the war.
Okay, first of all, the myth you quote from the Fiendish Codex I is just that - a myth, as told in one of the in-character sources the Fiendish Codex I uses. It's not "the truth," but just one of the legends the PCs will encounter. Because hardly any books mention the baernaloths (who have hidden themselves beyond the reach of most sages), alternate origins of the fiends have been posited. This is one of them.
Secondly, the myth in question suggests, as other sources do, that there was originally a single plane of Chaos. This plane was not the Abyss, and it wasn't Limbo either; nor was it Arborea, or Ysgard, or Pandemonium. It was all five planes of Chaos at once, completely undefined, completely wild, without borders or any lasting form.
In the dawn age before time began in earnest, the primal forces of Law encountered the primal domain of Chaos, and things began to change. The beings of Law (remembered as the Powers of Creation or the Twin Serpents, though neither title is completely accurate) took the stuff of raw Chaos and shaped it, creating the neutral planes. In shaping it, they also contaminated it, polluted it. They took what was most appealing in Chaos and what was ugly and deformed they left behind. The parts of Chaos that were polluted by their neglect and indifference, that were abandoned because of their exalted aesthetic sensibilities, that were twisted in hatred at these invaders who stole what was most beautiful from them - these parts became the Abyss. The festering hatred of Law, the hideous miscarriages of Chaos suddenly aware of itself - these things became the first of the obyriths.
These hateful remnants of what was once pure Chaos, primal and whole, became beings like the Queen of Chaos and Obox-ob, the first rulers of the fragmented, shifting realms of contaminated Chaos that would become known as the Abyss. It's very likely that the primal beings of Evil - the baernaloths - took advantage of the resentment they felt to corrupt them further, to layer in patterns of their own, to seed them with the larvae made from the discarded chaotic taint of their own children in order to ensure that the Abyss as as much a thing of evil as it was of Chaos.
And so yes, the true slaadi were probably the first inhabitants of the primal plane of Chaos. The obyriths are those, according to the myth you're quoting from, who appeared afterwards, after Chaos had been raped and violated by the forces of Law. Some of the first obyriths, like the Queen of Chaos, might have been originally slaadi before they fell into corruption and madness.
Throughout the history of D&D, there have been writings that seem to equate Law with Good and Chaos with Evil, despite the fact that this breaks the Alignment system. This stems back to earlier fantasy fiction that would talk about conflicts between Law and Chaos without mention of Good and Evil; in these works Law was always depicted as preferable to Chaos, and therefore Good.
This bias can be seen throughout D&D, and should probably be systematically purged by any Planescape DM. For example, the Paladin the archtypal champion of all that is Good must also be Lawful. Aasimars who are descended from Celestials of all alignments even get the Paladin as favored class. My suggestion? Use the variant Paladins from UA, and tweek them to be paragons of both of their alignment components, and not just one.