Brief rundown of the story of Arthur which applies to both Myth and Anime.
Arthur was son to Uther Pendragon but it was sort of hush-hush as he was a illegitimate child from one of Uther's servants wives. Merlin told Uther of a prophecy saying that he must have a child to succeed him in the throne which Uther believe but Arthur was not the child he desired (in the animes case, its because it was a female) Uther left Arthur in Merlin's care as to protect the successor to the throne. Arthur was then raised as a knight to live up to the king. Being raised on honor and duty, that which should befit a king.
Arthur after years of knightly training eventually comes to the Sword in the Stone (Caliburn) and Merlin tells her that if she draws the sword that she will no longer live as a human but that of a king and must cast aside any emotions. Arthur then drew the sword and was marked as King.
Arthur was like a god on the battlefield and never knew defeat, but eventually during one of the battles she was victories but Caliburn was broken during the battle. Merlin then went to see the Fairy Goddess of the lake Vivian (Lady of the Lake) who then forged a truly divine blade that surpassed even that of Caliburn as Caliburn was born from the earth while Excalibur was crafted in the realm of fairies. It was also forged with a scabbard known as "Avalon". Merlin brought the sword and sheath upon Arthur and asked which the king prefered. Arthur said the Sword but was scoulded by Merlin saying that the sheath is much more beautiful. It is better to never draw your sword and live protecting than to fight basically. Avalon also had within it a omnipotent power of immortality/eternal youth to the wielder.
Behind the scenes of all this Morgan was jealous and hated that Arthur a illegitimate child was given the throne over her of pure blood. She devised a plan to remove Arthur from the scenes by having a child with him. (anime version she just uses arthurs DNA to do this) Mordred hid his identity from everyone and become one of the knights of the round table who truly admired Arthur but became twisted from the words of Morgan which he eventually rebelled against Arthur. Avalon was stolen from Arthur which removed his immortality and during the battle of Camlann which was Arthur against the rebel group lead by his son Mordred where both Arthur and Mordred had killed one another in battle.
The anime takes this part a little different in which Excalibur was still keeping Arthur alive, there alone on the battlefield having no survivors but only Arthur. Killing her own son destroyed her inside, she was badly wounded by a final fatal blow from Mordred before his death but did no die because of Excalibur. Bediveri athurs most trusted knight aside from Lancealot came rushing to the battle finding his king badly wounded and he knew that the sword alone was keeping her alive she then told him to return Excalibur to the lady of the lake, he refused to do so but she put his loyalty into question which he couldn't refuse at this point. He eventually takes the Excalibur and returns it to Vivian, once he returns to his King who is lying next to a tree she whispers something to him and then she dies. <3