The hollow hills - Mary Stewart

Born on Christmas Eve, Arthur Pendragon is the illegitimate son of the married duchess Ygraine and Uther Pendragon, brother of a great king: a union Merlin arranged. But Arthur's birth is not welcomed by everyone: his father demands he be hidden until a legitimate son can be born, and Merlin is forced to give the babe to his own childhood nurse for safekeeping and go into hiding.

In a dream, Merlin learns of a sword of unparalleled beauty and extraordinary craftsmanship, hidden in a long-deserted Roman temple. Merlin retrieves the sword and hides it in a cave on an island in the centre of a lake, where it remains for years.

But what is hidden must be revealed. For it is foretold that the irrepressible young man named Arthur will find a sword and claim his birthright: as heir to Uther Pendragon and king of the Britons.