Lake Salisbury

is a lake, known for having the highest concentration of magic power in all of Britannia.

Background
A legend says that a princess who lives in the lake bestowed a mysterious sword to a human king. This legend actually refer to the Lady of the Lake, who gave the sword Excalibur to Carfen, a master swordsman.

Demon King arc
After take possession of Zeldris's body, the Demon King goes to the magic lake of Salisbury in order to recover the magic that he lost in his fight with the Seven Deadly Sins. There, he waits for Meliodas who arrive in company of Elizabeth. A fight is then unleashed between them, with the other six Sins joining eventually. During the fight, the Demon King gain access to an unlimited magic source provided by the lake. Thus, Diane uses her Mother Creation to destroy the surrounding terrain and force the Demon King out of the lake, destroying most of it in the process.

All the water of the lake maintained acumulated at the end of the a pit. When Meliodas destroys the Demon King with a combined attack from all the Sins, Merlin instructs Meliodas in direct the attack to the lake in order for it to absorb all the excess of power from their attack and prevent it from destroying Britannia.

King of Chaos arc
When Merlin decides to reveal her true goal, she teleports the Sins and Elizabeth to Lake Salisbury where they find Hawk and Hawk Mama in front of the orb of water that used to be the lake. Merlin then says it is time to start, when Meliodas asks her what, Merlin surprises everyone by declaring that she refers to Arthur's awakening.

Upon putting herself over the lake, Merlin removes Arthur's body still pierced by Excalibur from the Boar Hat. She says that all the preparations are set and asks the "princess" if she is ready, to which a voice from the lake asks if it is finally time to meet their master. As the lake water begins to shine and its light reaches Excalibur, Merlin tells Arthur that she was wrong since the sword was not the one that would kill him but the key to advancing to his new stage. Merlin uses the "key" to unlock the door.

After an explosion of light, Arthur wakes up having revived as the "King of Chaos". After awakening, Arthur is overcome by pain and when Meliodas comes to his aid, he still believes the Dragon's Sin to be in his Assault Mode and claims he won't let the Demon Clan have their way with Britannia, causing his power to activate and break down reality.

After calming Arthur down, Merlin explains to Arthur the reasons behind Meliodas' "betrayal" and the latter realizes his mistake but is glad to see Meliodas back to his old self. But when Meliodas asks Merlin what Arthur just did, the same voice from before speaks up and introduces herself as the Lady of the Lake, who Arthur recognizes as the one who gave King Carfen the holy sword thousands of years ago and claims to be wrought from the entity Merlin refers to as "Chaos".

Meliodas, however, doubts the entity's existence as he has never heard of it once during his 3,000-year journey across Britannia and the Lady of the Lake explains that is because Merlin purposely hid it from him, Elizabeth, and the others and when Meliodas asks for an explanation, she tells everyone of Merlin's lonely childhood as she never received any form of love from anyone until she heard the legends of the entity known as Chaos.

The legends state that Chaos first created the world and all life out of nothing, before creating the Demon King, Supreme Deity, and Sacred Tree as its "offspring", who in turn created their own realms and the first three races: the Demon Clan, Goddess Clan, and Fairy Clan.

Furthermore, Chaos also created the Giant Clan and Humans before being sealed by its first two "offspring" out of fear and envy of their creator's power before sealing it away in a transient form, revealed to everyone as none other than Hawk Mama, who begins to deflate to everyone's shock before Merlin explains that it's content, Chaos, has left its shell and made Arthur its new host, to the young king's shock upon believing the "giant green pig" is inside of him despite Merlin claiming that's not technically the case.

Not wanting to see Meliodas and Merlin argue over him, he explains how he always looked up to the Seven Deadly Sins as a child, while unconsciously using Chaos' power again and claims how he wanted to be a magnificient king and finally realizes how he unconsciously changed the world around him to his immediate shock and confusion; which is immediately followed by Cath revealing itself as Arthur is glad to see his friend only for the cat-like creature to attempt to devour Arthur, only failing due to Meliodas' intervention.

However, it's revealed that Cath succeeding in tearing off Arthur's right arm and devouring it to everyone's horror as the Lady of the Lake fearfully reveals Cath's real identity as Cath Palug, a tyrant beast born from Chaos who desired its creator's power but failed and was later sealed in Purgatory by the Goddess Clan for its misdeeds. Meliodas and Ban are ready to attack but Arthur disbelieves Cath's betrayal to be true only for the creature to remorelessly reveal its true intentions.as it then uses the fragment of Chaos in an attempt to devour Arthur and the others.

In the outside world, the Lady of the Lake senses a disturbance in the pocket dimension created by Cath and it is broken as she reveals that Excalibur, having fulfilled its duty as the "key", has now awoken as the true Holy Sword and upon seeing Cath for the irredeemable monster that he is, Arthur is able to strike him down and seemingly kill the monstrous creature before collapsing from exhaustion, with Merlin then sending everyone else back to Liones out of guilt for using her friends and claims she'll protect Arthur even if it costs her life.

Fights and Events

 * Seven Deadly Sins & Elizabeth Liones vs. Demon King

Trivia

 * The legend mentioned in the story is a reference to Lady of the Lake from the Arthurian Legends. This character is the one who gives Arthur his iconic sword, Excalibur.