Aurelius is the only living son of Arthur and Guinevere, born after his father’s death in Camlann, and raised in a monastery, unknown to all in Britain. He knows nothing of the life of the court and even less of the life of a warrior. An aging Lancelot has been called to train him—his mother’s dying wish.
But Aurelius cannot use the sword, is terrified of horses, and wants only to escape his father’s name and the burden it places on his future. Lancelot is killed because of his (whose??) cowardice and Aurelius and the girl he loves, Iseult, are enslaved by Irish raiders. Iseult is raped, and when he kills the rapist in a berserker rage, Aurelius becomes a hunted fugitive.
In this final extremity, Aurelius allows his natural talent for leadership to emerge. He finds a group of bandits, all British slaves, who begin to depend on him. Then the group grows, and soon Aurelius is conspiring in a massive slave rebellion. He saves hundreds, and returns to Britain on a ship, ready at last to become High King of Britain.
At the end of this first book, he writes of himself, “I am Aurelius Ambrosius. The words mean eternal golden one. I am neither. I am the son of Arthur, my life taken from the first moment of my breath, by expectations, by fortune tellers and soothsayers, by gods and magic, by my father’s ghost. Free will, the bishop promised me. But I have never been free.”
There's nothing new here. You need something to hook my interest beyond "Arthur's unknown son". That's just a gimmick to launch the novel. You actually have to write something that brings fresh perspective to this old as the hills story.