Seven Deadly Sins & Arthur Pendragon vs. Cath Palug

Seven Deadly Sins & Arthur Pendragon vs. Cath Palug is a battle between the Seven Deadly Sins and Arthur Pendragon against Cath Palug. It is also the final battle in the series.

Prologue
After returning to Liones following the Demon King's defeat, Diane asks Merlin what her goal was and she decides to show the others by teleporting them back to Lake Salisbury where she begins Arthur's "awakening".

Using the magic in the lake alongside Excalibur, Arthur is brought back to life but is different as he appears to be in pain and when Meliodas tries to help him, Arthur begins to unconsciously alter reality itself due to his emotions.

Merlin calms Arthur down and the others explain to him the reasons behind Meliodas' actions to his surprise and guilt for jumping to conclusions. However, when Meliodas asks Merlin what happened with Arthur, she explains it was the power of Chaos, an entity that created the world, the gods, and the five races from nothingness.

Tensions begin to rise between Meliodas and Merlin as the former realizes that Merlin was secretly pulling the strings behind many of the conflicts that they have dealt with and accuses her of using both the Sins and for using Arthur as a means to an end, but he interrupts saying they should not argue over him, once again unconsciously using Chaos' power and suddenly, Cath appears from the Boar Hat and shows relief to Arthur's safety.

Initially happy to see his mysterious companion, Cath suddenly attempts to devour Arthur but due to Meliodas' intervention, only succeeds with ripping his right arm off and devouring it, thereby absorbing a fragment of Chaos.

Battle
Everyone is shock and horrified as Meliodas and the others try to help Arthur while the Lady of the Lake explains Cath's origins as a tyrant beast called Cath Palug, who desires the power of Chaos for himself; the reason why he protected Arthur until his power awakened so he could devour him and take the power from him.

Arthur, despite his pain, believes it to be a misunderstanding but Cath reveals everything to him and says he was never Arthur's friend to his horror as he attempts to warp space in order to devour Arthur (and the others by extension), but Arthur uses the newly formed Excalibur to destroy the dimension Cath created, to the creature's delight upon seeing the power of Chaos firsthand and begs Arthur to let him eat the young king.

Showing relief that Cath is an irredeemable monster, Arthur claims he can kill him without hesitation and responds by slicing Cath in half, seemingly destroying his body and defeating him, before collapsing out of exhaustion and is caught by Merlin.

Elizabeth and the others are sent back to Liones by her, believing she no longer deserves their kindness and brings Arthur into one of the castle-like buildings he unconsciously created to rest but as he wakes up and talks with her about her plans, the tower begins to shake and Cath's voice is heard calling for Arthur as he was not destroyed by Arthur's attack and appears in his true form, claiming he will not lose a second time.

Merlin attempts to protect Arthur from the cat monster but she is easily pushed back and when Arthur comes out to deal with Cath himself, the tyrant beast gleefully tells Arthur of Camelot's destruction in the fight between the Seven Deadly Sins and the Demon King.

Arthur is brought to despair upon learning of his kingdom's fate, giving Cath the chance to attack but is suddenly reflected by Meliodas' Full Counter and Merlin is shocked to see the others return, refusing to give up on her as well as Arthur, claiming they will take responsiblilty with her and help her protect Arthur, to her joy and gratefulness.

Cath is annoyed that the Sins have returned to help Arthur, who regains consciousness and Cath tells Arthur it is fine because everything eventually dies and Arthur believes he is trying to tell him there is no point, but is cut off by Meliodas and the others who refuse to let the monster have Arthur.

Soon, the beast charges and the Sins, Elizabeth and Arthur all take on the beast, but due to the fragment of Chaos, is now beyond life and death and uses the power to trap them each in a vision of a possible future shaped by their desires, but Arthur and Meliodas manage to break free out of willpower and realize that Cath has the advantage since they can not kill him, to which he revels in as he believes victory is his; Arthur then realizes there is a way to defeat Cath without having to kill him.

Ridiculing Arthur for claiming to have a way to defeat him, he warps reality to devour them all but Arthur turns the table by claiming he will devour Cath instead and an even larger vortex appears from Arthur's will and is able to return everyone else to their senses.

Cath attempts to reject Arthur's pull while Arthur claims he will absorb Cath if he can not defeat him and says he will be taking back the fragment that Cath stole, to the beast's refusal as he tries to reject even further. Arthur asks Cath why he wants Chaos in the first place and Cath states all he want is destruction itself, to erase everything with a form; Merlin realizes that Cath himself is the manifestation of the destructive will wrought from Chaos.

Asking what Arthur would use Chaos for, Cath claims it would simply die off sooner or later, that any kingdom he attempts to rebuild with fall again and tries to guilt Arthur by saying those of his subjects who perished in the Holy War must hate him for not being able to save him, but Meliodas' words are what give Arthur hope and strength to delcare that he does not mind if his subjects hate him or mock him, as long as they are happy and safe - an everlasting kingdom where nobody has to suffer, is what he claims to be his goal.

Cath, however, claims Arthur to be crazy by declaring something like that as he believes that nothing is permanent in the world, that is a contradiction; Chaos itself. Unfortunately, by saying that he realizes he fell into Arthur's trap as the latter declares himself as the "King of Chaos" and begins to absorb Cath into himself and reclaims the complete power of Chaos, thus returning the world to its natural state.

Aftermath
With the threat of Cath gone, the world is returned to normal and everyone finds themselves in the ruins of Arthur's chaotic castle in the middle of the night.

Diane asks what happened to Cath and Arthur believes him to be somewhere inside of him, though quite unsure and wonders if he has the full power of Chaos again to which Merlin confirms after she tells Arthur of the Holy Sword that has transformed into a new state with attributes very similar to the Goddess Clan, reflecting the state of Arthur's heart in a positive manner after becoming confident and hopeful.

Meliodas comments on Arthur's new power, claiming he will have no problem with his goal now, to the young girl's flatterment as he begins to feel unsure of what he said, but Meliodas tells him he can not back down now that he is spoken it aloud to the world and tells him if he ever goes off his path, the Sins will be there to help, to his gratefulness.