Uther Pendragon was king of Britain and wanted nothing more than the love of Igraine, but she wanted no part of him. So he went to the wizard Merlin, who promised Igraine would have him if he agreed to give Merlin his first born.
This shouldn't be too hard!
Seeing as the kingdom couldn't be ruled by a child, most nobles tried to make themselves king, creating conflict, confusion, and agony for the people.
You're right; that was too hard at all!
Merlin took the child to a knight named Sir Ector. Here, Arthur would be raised alongside the knight's son, Kay.
I shall bring all justice and what they deserve!
After years of training as a knight, Arthur travelled, with Sir Ector and Kay, to Canterbury. There, Merlin had been instructed to devise a test that would prove to be the rightful King of Britain born. All a man had to do was pull the sword from the stone.
No one but Arthur could pull the sword free. And when he did, he proved himself as the rightful king.
But Arthur had to prove what he had done before again. The Archibishop requested that he replace the sword and remove it again. After multiple tests of the validity of Arthur's achievement, he still remained as the only man who could wield the sword. So it was set, Arthur would be King of Britain.