Guardian of the Balance (Merlin's Descendants, Book 1)

Her name was Arylwren - Wren to those she numbered among her friends. She was the child who should not have been, daughter of Myrddin Emrys - The Merlin - and Deirde - The Morrigan - high priestess of the Druids. Both had been promised to the gods, and Myrddin, gifted with prophecy from childhood, has been sworn to eternal celibacy in exchange for the Sight.
As a child Wren roamed the land with her father, the two welcomed everywhere for the bardic tales and the information they brought. Thus The Merlin could secretly watch over his other charges: Arthur, who would one day be King, and the other youths who would prove Arthur's most loyal followers. During their travels Myrddin taught his daughter of the old ways and the small magics and bardic skills.
But even The Merlin could not foresee the power with which Wren had been blessed. Nor did Wren tell him of the visions which had been given to her alone: images of war to come, of a forbidden passion brought to fruition, and of those whom she loved in peril of being devoured by all-consuming forces of darkness. She only knew that she must learn to master the talents she had been born with, for a time would come when she must become both sword and shield for The Merlin, for Arthur, and for the people who would be put in her charge.