The Crystal Cave (The Arthurian Saga, Book 1) by Mary Stewart

Born the bastard son of a Welsh princess, Myridden Emrys -- or as he would later be known, Merlin -- leads a perilous childhood, haunted by portents and visions. But destiny has great plans for this no-man's-son, taking him from prophesying before the High King Vortigern to the crowning of Uther Pendragon . . . and the conception of Arthur -- king for once and always.