From the starting of the series it is not very well described about what is so special about Arthur. The series starts off from a newborn Britannia, which is a result after the war ended between the Demons and Goddess which is also known as Ancient War as it is described in the starting of the series.
In the series it is clearly shown that Arthur is the perfect balance between good and the evil. So, this is the reason that Merlin is so close to Arthur and she wants Arthur to ascends to the God of Chaos. Which is also her aim to bring Chaos into this world from the very beginning.
Holy Knights which are introduced into the series are a reference to Round Knights and most of the characters are based on Merlin and Arthur.
Coming to the main point in the story goddess clan have been wiped out and fairies amd giants are not in abundant like ago after the ancient war but the only species left is human beings.
So, coming to the sealed demons they have become a myth after the ancient war. The demons became to known only after Hendrickson discovered them and after that the ten commandments break free from the seal.
Arthur had the duty to unify and protect Britannia and Merlin was responsible that Arthur acheives these tasks without any problem. Merlin main task was to protect Arthur who will lead this new era. So, that's why Merlin is so close to Arthur.
Do Arthur die in Nanatsu no Taizai?
Well spoilers alert!
Yes, Arthur dies during the end of the arc in Nanatsu no Taizai( Seven Deadly Sins). Well it is weird as the manga has approached its ending and there is no reason to keep Arrhur alive and another thing is that from the starting of the series it is teased that Arthur plays a big role in the coming arc but it doesn't happen.
It is also weird when we think that Seven deadly sins is based on Arthurian legend, as well as it was also teased by the author the manga that after the ending of the series a sequel will be introduced where Arthur will be the main protagonist. The story was going to based on Arthurian legend, Pendragon and knights of the round table which is shown as Holy knights in the series.
It would have been cool if Arthur was given a sequel as Arthur is most likely a main character and he always admired Meliodas and wants to be like him. So, it can be said that Arthur as a character is very interesting but not very well exploited as it needs to be.
So, at the moment it can't be said that there will be sequel of Seven deadly sins which will involve Arthur as currently there is no planning of doing so. And Round table of Knights had not been created yet. In the legend it is shown that Tristan is the son of Meliodas and Elizabeth and he is also the member of Round Table.
In the Arthurian legend it is shown that Meliodas is also the member of the Round Table and in the series Seven Deadly Sins it is shown that Meliodas agreed to be the part of the Round Table. So, there are a lot of similarities between the two.
Between Meliodas and Escanor, Which Character is more stronger?
Well to be honest Meliodas seems to be more powerful than Escanor. As, Meliodas is most likely to be the next demon king. Meliodas will be the vessel for the Demon king. But in reality powers of Meliodas have surpassed the expectations of the demon king itself. Meliodas had always been a strong character even from the starting of the seven deadly sins ehen Ten commandments had to group together in order to defeat Meliodas.
The difference is clearly seen when Zeldris exercise the powers of demon king but Meliodas powers far surpassed that of his. In return Meliodas will lose all his emotions and memory for using that power and that is the price he had to pay.
So, vice verca the more emotions he gains more likely his power is to be diminished. Another reason aside from this to not use this power is that he will have to return underworld to his dad the Demon king and will have to leave Britannia which he doesn't want to do.
But, Escanor is also not weak as he is one of the strongest character in the Seven Deadly Sins. The power that Escanor wields is Grace that once a time belongs to the angel Mael. When Mael transformed into demon by the Demon King he was given commandment by him and in this process he lost Grace Sunshine which now belongs to Escanor. So, Escanor is the only one who comes close in power level comparison as he possess the most strongest Grace of the archangel Mael.
But there are also risks associated with it as the more Escanor will use this power his power will consumed his body more which will eventually leads to his death. He is not able to fully utilize Sunshine as his body can't handle it. This we had seen whem Mael use Sunshine his power level was far beyond as compared to when Escanor uses the same.
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As we reached to the end of the series Mael stated that Escanor have become very powerful and his power level have come closer to that of the Demon king. When Escanor defeated Meliodas before when he used The One technique, his power level surges to an extreme level where he was able to defeat Meliodas who in his demonic form and he overpowered the Seven deadly Sins other characters.
During the last fight with the Demon king Escanor unleashed all his power for a short while which took a lot if toll on his body. So, it can said that Escanor and Meliodas are same in power level but Meliodas somewhat surpasses Escanor.