Chaos

Chaos (カオス, Kaosu) is explained as the force that was considered the darkness of the Amatsu-Mikaboshi. Upon the latter's destruction by the movements of Yin and Yang, this force had echoed throughout the newly established universe and is said to now exist in all beings. It is theorized to be the force responsible for causing souls to become Hollows. The Ichiken Kyūkage of L'Obscurité are said to each represent a method at which chaos can drive people to utter madness.

Overview
It is a force that is generated from the hearts of those who experience emotions such as rage, anger, and hatred. These emotions become materialized into actual wavelengths that emit from the souls of those beings whom experience the emotion in question. Chaos causes the minds of those subject to it to become lost, becoming both senseless and unpredictable, much like pure Hollows themselves — the incarnates of chaos.

Yōryoku serves as a natural facilitator which fuels the force of chaos in those who are able to wield it, meaning that those whom use the Mukai generate a level of chaos force so immense that it has a significant effect on those around them. One unique case of a being's chaos being amplified by another power is Seireitou Kawahiru, whose inner chaos was roused and magnified to almost god-like levels by his own Zanpakutō. This change in persona was later coined as "Shiroyasha", the same moniker given to Seireitou during the war.

The main factor which appears to influence chaos is emotion and desire. Sōsuke Aizen noted that the chaos force itself could be extracted from the souls of spiritual beings and modeled into a pure manifestation, having conducted several experiments to test this theory and eventually came upon the conclusion of "Hōgyoku" - a tool which can turn the very desires of whatever souls it is focused upon and materializes them into actuality given that they have the power to truly allow it to happen.

Chaos appears to have the ability to actually cause beings subjected to its force to undergo transformations. However, it depends completely upon the degree at which chaos has been operating from within them and what factors are involved in such an occurrence.

It has been explained that everyone has some degree of chaos within them, though some far exceed the normal amount. Kamui noted that the Sōzōshin in particular have abnormal levels of dormant chaos within them that leads to some of them developing their psychotic instincts in combat. Below them, Hollows are said to also possess incredible chaos within them as well. While to a lesser degree, Shinigami and Humans also possess some level of chaos within.

Domino Effect
"It is said that the force of chaos can rouse the insanity that rests deep within all souls, causing it to become involuntarily magnified. Quite literally, it spreads like a disease."

- Kamui

Behind the Scenes
The concept behind Chaos lies in both actual japanese mythology regarding the Amatsu-Mikaboshi, but its true inspiration is drawn heavily upon the idea of Madness from the anime and manga series, Soul Eater.