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 before creating the Supreme Deity and the Demon King, then later created the Sacred Tree as it's third "offspring". 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 which it put everything into: 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.

Due to Chaos not being 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.

At some point, it was challenged by the power-hungry Cath Palug for it's power but it easily defeated it.

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

When Merlin decides to carry out Arthur's awakening, she teleports to Lake Salisbury along with 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.

After transforming, the Lady of the Lake reveal Cath's true identity as Cath Palug, a monstrous and tyrannical monster that once challenged Chaos for its power but easily lost and was later sealed by the Goddess Clan for it's misdeeds. Arthur asks Cath himself about this to which Cath fully admits it to be true and that he only clung to Arthur until he became the new host of Chaos so the beast could devour him. He attempts to devour Arthur, along with the Sins, Hawk, and Elizabeth in a dimension using the fragment of Chaos' power it devoured, but Arthur uses Excalibur to cut Cath Palug in half, destroying its body and killing it.

Arthur is relieved to see the threat has passed, only to fall unconscious and be caught by Merlin, who thanks Elizabeth, Meliodas, and the others for their help but says they shouldn't be together anymore and takes Arthur away with her, vowing to protect him with her life.

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 became the new host of Chaos and is able to use the power of Chaos to deform reality itself.

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.