Mary Stewart's Merlin trilogy.

A trilogy plus one. Book One: The Crystal Cave, Book Two: The Hollow Hills, Book Three: The Last Enchantment, Book Four: The Wicked Day.

Book One: The Crystal Cave. From a young child, a bastard with an unknown father, we watch Merlin grow from a shy child to a young man who discovers and claims his power as the close council to Kings of Britain. From the dust jacket, "Into this strange world was born Merlin, bastard son of Niniane, daughter of the King of South Wales, and an unknown father. The novel opens in Wales when Merlin is seven, and closes in Cornwall, at Tintagel, with the begetting of Arthur."

Book Two: The Hollow Hills, Merlin watches over the child Arthur [yet another bastard with an unknown father] and guides him to his destiny. From the dust jacket, "... this tale of Arthur's growth into manhood and his discovery of the strange sword that was to test his claim to power."

Book Three: The Last Enchantment. Merlin and King Arthur's stories of warfare, peace, love and loss.

Book Four: The Wicked Day. Mordred's story. King Arthur's illegitimate son grows up ignorant of his heritage. Around age 12, he's summoned to court by his father, learns of his status, deals with his step-brothers and is ultimately his father's bane.