Chaos

is an impure entity of darkness that even the Demons feared and a pure entity of 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 this was easily defeated.

King of Chaos 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 of power, so it does not manifest itself in physical form and that "Hawk Mama" was just a provisional form in which the two gods sealed it. 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 sends them back to Liones and vows that she will protect Arthur with her life.

After sending the Sins back to Liones, Merlin brings Arthur into one of the castle towers he created unconsciously and is overcome with joy upon seeing only a mere glimpse of the power of Chaos, just as Arthur awakens as is glad to see her in a better mood. She asks him why he isn't angry with her as she only revived him for her own desire but Arthur says that despite Chaos' origins and how powerful it is, as long as the one controlling it has good intentions, it could be used to make the world a better place, his positive emotions reflecting that onto Excalibur.

Suddenly, the tower begins to shake and a voice calls out to Arthur, saying it's not done playing with him yet. Arthur and Merlin both recognize the voice belonging to Cath Palug, who survived Arthur's attack and has further transformed into a more powerful and stabilized form. Arthur is shocked, believing he had destroyed him but Merlin claims she won't let Cath get to him as she goes outside and attacks him, claiming she will be it's opponent.

Merlin struggles against the monster, who still possesses a fragment of Chaos from Arthur's arm as he says that she isn't the one he wants to play with, only for Arthur to appear and saying he shall do it for her. He cuts the monsters into pieces and says it's his fault he allowed Cath Palug to get this far and claims it is his responsibility to defeat it, but the monster keeps regenerating and says cutting it won't kill him, but Arthur claims he will do whatever it takes as it's his job to protect Camelot and it's people.

Cath Palug mocks him claiming he doesn't realize he's already lost them and explains, to Arthur's horror, that Camelot was destroyed in the fight between the Seven Deadly Sins and the Demon King in Meliodas' body and despite Arthur's intial disbelief, Cath says he can prove it by using Chaos to see and Arthur's fears are realized upon seeing that his kingdom was wiped off the face of Britannia.

Merlin tries to tell him that he can rebuild Camelot and that most of the people were evacuated to Liones, but Arthur is greifstricken upon realizing the people in the underground vault were killed, but Cath takes the opportunity to attack Arthur, only for a barrier to form around him and is attacked by Merlin who attempts to kill her with a breath attack, only for Meliodas to appear and use Full Counter to save him, much to her shock.

Elizabeth and the rest of the Sins appear and claim that Merlin needs to take responsibility for unleashing Chaos, and as her friends, need to let them carry some of the responsibility by protecting her and Arthur from the monster, to her gratefulness.

Cath reforms again, annoyed at Meliodas and the other's interference and tells Arthur to simply give up as nothing will last forever, but Meliodas, Ban and the others refuse to give Arthur up to the monster cat who begins its charge and uses the fragment it stole from Arthur to show them visions of a possible future of their own desires but Meliodas manages to break free and Cath mocks them, saying they cannot kill him as he now trascends life and death, but Arthur realizes there is another way to defeat him.

Arthur states that since he can't destroy Cath, he will simply devour him instead and absorb him, much to Cath's amusement as he attempts to devour Arthur and the others by using the fragment of Chaos to manipulate the environment, but Arthur creates an even large vortex which manages to free everyone from their visions as he asks Cath why he wants Chaos so badly, only for Cath to reveal itself as an incarnation of the destructive will of Chaos that only wants to destroy anything and everything and tells Arthur that it won't matter even if he rebuilds Camelot as it will only fall again and his subjects that he failed to save are probably cursing him in death for being a failure.

However, Arthur counters this by saying despite what others may or may not think of him, as long as he can make them safe and happy, it's fine if they laugh at him or talk about him behind his back as he claims he will make an everlasting kingdom where nobody has to suffer or feel fear; Cath finds his notion ridicolous as it is a contradiction and realizes that he fell into Arthur's trap by saying those words and devours the monster into himself, declaring himself as the "King of Chaos".

After absorbing Cath into himself, the dimension it created disappears and they find themselves back in the normal world during the night and Merlin explains that by absorbing Cath, Arthur has regained the full power of Chaos and the young king is shocked to see Excalibur transformed again, now in it's final form. Meliodas admires Arthur for both his new power and promise to make an everlasting kingdom. Arthur tries to shrug it off as him being boastful, but Meliodas reassures him that he's set his goal and claims that the Seven Deadly Sins will always be there to help him if he falls off his chosen path and Arthur thanks them, claiming them to be his everlasting goal.

Abilities/Equipment
Chaos is shown to be an extraordinary powerful being, as it created the world and immensely powerful god-like beings, such as the Supreme Deity, the Demon King and the Sacred Tree out of nothing on a mere whim, that in turn were each capable of creating their own powerful races to serve them and own unique dimension to reside in.

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

When Arthur became the "King of Chaos", he became the new host of Chaos and is able to use it's power to bend reality to his will, albeit unconsciously, and create new worlds.

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.