Chaos

is an impure and pure entity of darkness that even the Demons feared and light that even the Goddesses worship.

Appearance
Chaos is a will, therefore it does not manifest itself in a physical form.

Personality
Nothing is known about its personality, except that it was disappointed with it's creations prior to creating humans, due to them not living up with its expectations yet saw humans, despite being incomplete and much more fragile and short-lived than the other races, as it's greatest creation due to being the closest race to mirroring Chaos.

History
Said to be born amidst ancient stars that were very far away, Chaos first created the world, then it bore that Supreme Deity, the Demon King and later the Sacred Tree. After that, it created the fourth race, the Giants. Chaos was disappointed with them because they were a savage race, slaves to their power and knowing nothing other than inflicting pain on each other. Then it created Humans, kindness and malice, bravery and cowardice, love and hatred, beauty and beastliness. Although incomplete, they were the perfect race that came closest to it. The Supreme Deity and the Demon King were filled with jealousy and apprehension of Chaos, fearing that the love and respect that their own races had for them, would be stolen by their own creator. Thus, the two gods joined together and sealed Chaos away, allowing them to be the two rulers of the world and believed to be the most powerful entities in existence.

Since Chaos is not a physical entity, it was not truly sealed. It comes to exist within a shell that would be known as the Mother of Chaos and, later, Hawk Mama.

Current arc
After the Demon King was destroyed by the Seven Deadly Sins, and the balance between light and darkness was lost, Merlin finally got the chance to revive Chaos.

When Merlin decides to carry out Arthur's awakening, she teleports to Lake Salisbury along the Sins and Elizabeth. Upon putting herself over the lake, Merlin removes Arthur's body still pierced by Excalibur from the Boar Hat. So, thanks to all the magical power that the lake had accumulated in the battle against the Demon King, Merlin and the Lady of the Lake manage to awaken Arthur as the "King of Chaos".

When Meliodas reproaches Merlin for using the Seven Deadly Sins to revive a being that may never exist, the Lady of the Lake states that Chaos exists and that they have already been face to face with it for a long time. When no one seems to understand, the Lady of the Lake reveals that they have been traveling on it and that all that time has been waiting for her resurrection; leaving everyone shocked to see that she refers to Hawk Mama. The Lady of the Lake says that Chaos is a will, so it does not manifest itself in physical form and that what the two gods believed to seal was not really Chaos. There, Hawk Mama is revealed to be a moss shell whose content, Chaos, is now inside Arthur. Merlin states that Chaos has chosen to coexist within Arthur, which proves that he is the true King of Chaos

When Cath shows itself happy to see Arthur, the Lady of the Lake is alarmed by its presence and tells Merlin not to let it approach Arthur. Cath then devours one of Arthur's arms and when it transforms into a monstrous form, the Lady of the Lake tells them that they must retrieve the arm or they will soon know how powerful even a fragment of Chaos is.

Abilities/Equipment
Chaos is shown to be a extraordinary powerful being, as it created the Supreme Deity, the Demon King and the Sacred Tree from nothingness, and the three in turn created three of the most powerful clans in the world and three separate dimensions to inhabit and live in.

It required the combined godlike power of both the Supreme Deity and the Demon King to seal it away, making it the most powerful known entity in the entire world of Nanatsu no Taizai.

When Arthur became the "King of Chaos", he was able to use the power of Chaos to deform reality.

Trivia

 * Chaos is likely based off the concept of the same name from Greek mythology, said to be a void state that existed before the world itself and was said to have spawned the greek gods from nothingness, similar to how Chaos did the same with the Supreme Deity and Demon King.