The story of King Arthur begins properly like most hero myths, with his birth. The birth of Arthur is of typically mythic proportions, with plenty of the supernatural involved. While most heroes receive the divine treatment—most commonly due to Zeus—Arthur can’t go this route. Christianity has well and truly spread across Europe, even into the far reaches of once-Celtic Britain, and it would be blasphemy for Arthur to be sired by God. It can’t happen.
Arthur’s father is Uther Pendragon, but instead of sleeping with his own wife, he goes in to sleep with Queen Igraine, the wife of his enemy King Gorlois. The reason for this is because of Merlin, the wizard everyone has heard of. Merlin disguises Uther to look like Gorlois, and then Uther goes in to sleep with Igraine while she thinks that her husband has come home early.
And, yes, while we’re here, this is rape. It is not cool to magically look like someone else, fooling someone into sleeping with you. We expect this kind of thing from the Greeks, but not Christians. What can I say, it’s the Dark Ages. Also, this story is plagiarized.
If we set the Wayback Machine for Ancient Greece, we will find the story of Hercules, who was conceived by Zeus masquerading as King Amphitryon to sleep with his queen, Alcmene. Either Merlin knew his mythology, or Geoffrey of Monmouth did. Either way, this story has clearly been cribbed.
The result is to create someone synonymous with Hercules. To unify Britain under his rule, he will need not only great strength and physical prowess, but the intelligence, wisdom, and mercy that is the hallmark of Hercules.