The literary term that you are looking for is known as Anagnorisis. For Arthur, it would be when he was able to pull the sword out of the stone, something that no other person could do, he does it with ease.  Since the stone is inscribed with the description of the person who is able of pulling the sword from the stone, it is then that Arthur experiences a startling moment of true revelation.

"Anagnorisis (an ag NOR ih sis) In Greek drama, a startling discovery; moment of epiphany; time of revelation when a character discovers his true identity. 

 "One of the finest occurs in Sophocles ’Oedipus Rex when a messenger reveals to Oedipus his true birth."