All existence is in the process of becoming. The three domains of Creation, Destruction, and Purification are the fundamental and divinely determined forms of elemental matter and represent the full scope of that process of becoming as it is knowable by mortal minds. As "help cosmology" explains, there is such a thing as form without matter, or, in other words, domain-aspected aether without any trace of the four elements: that is what the gods are and what minds or souls, liberated from their bodies, become.
But the notion of matter without form is paradoxical. To even think or perceive something is to give it shape or seek a pattern. Absolute formlessness is literally inconceivable. Yet, if one reflects on Rithera's cosmology carefully, this formlessness is what the gods encountered when they first arose. There was supposedly *something* before form. What was that originary tumult that the gods, by their very natures, molded into the three domains? Is it gone forever, the divine Big Bang having passed? Or could it be made to return?
Chaos is elemental-aspected aether without any trace of the domains. It is not evil by the standards of mortal morals, but it is "godless," a rejection of the way the gods ordered the cosmos, and incompatible with the fabric of reality. It is thus nearly impossible to sustain and totally unknown to almost all mages throughout recorded history, and has no fixed appearance, its manifestation depending on its elemental aspect. To generate it, overwhelming elemental power is required, on par with the elemental Beasts that once ruled over the beastkin. They are not inclined to share its secrets, considering its use "blasphemy" against the gods who appointed those Beasts after the Primordial War. Unbeknownst to mortals, it was when the ancient beastkin fused their very bodies with aether gems seeking Chaos, the power to reorder and recategorize the cosmos itself in defiance of the gods' plan, that they were abandoned by their patron Beasts and their golden age ended.
Chaos and domain-aspected aether negate one another, like matter and anti-matter. Thus, in theory, Chaos could dispel light or shadow magic, rend a soul asunder, annihilate an eidola in an instant, or even destroy a Gate. But none of this has been attempted to anyone's knowledge, as no human has ever possessed sufficient elemental power, and the Forbidden Arts of the beastkin have been lost.
If a Loremaster were asked what the greatest threat to civilization is, they would likely respond "the Gates," which destabilize the world with their leaking power, twist the minds of mortals, and feed and foster eidola. If the Loremasters knew more of Chaos, the question might give them more pause. Where the Gates and the eidola corrupt the world by reshaping existence to suit their own forms, Chaos corrupts by undoing form altogether. In a sense, to the cosmos as the gods made it and mortals dwell in it, these preternatural forces are threats from opposite directions.