A five-book series of children's books by Susan Cooper, based on Arthurian legend combined with a mix of various mythologies. If you have not read these books, you might be wary of spoilers in the descriptions below.

The first book in the series, Over Sea, Under Stone, involves the Drew children, Simon, Jane, and Barney, in a search for the Holy Grail. They are aided by their mysterious "great-uncle" Merriman Lyon. Notably, this book is the only one of the series in which Will Stanton does not appear. Nor do the fully-conceived ideas of Old Ones, the Dark and the Light, and the other driving motives of the series. This is generally considered the weakest of the five books.

The second book of the series, The Dark Is Rising was a Newbery Honor Book. It introduces the reader to Will Stanton, and reintroduces Merriman Lyon. The Drew children do not appear in this book. Seventh son of a seventh son Will Stanton experiences strange things on his eleventh birthday, and is thus inducted into a world in which he is the last of the Old Ones, eternal guardians of the Light against the forces of the Dark, which include the fearful Black Rider. In order to defeat the Dark in the first battle of the final war between the sides, Will must recover the Six Signs of the Light: wood, bronze, iron, water, fire, stone (though not necessarily in that order).

The third book of the series, Greenwitch, brings the Drews and Will together for the first time. Together, the children must recover the Holy Grail, which has been stolen by the Dark. They must also recover an ancient manuscript, lost in the first battle for the Grail. This manuscript allows the cracking of a cipher-inscription on the Grail:

"On the day of the dead, when the year too dies,
Must the youngest open the oldest hills
Through the door of the birds, where the breeze breaks.
There fire shall fly from the raven boy,
And the silver eyes that see the wind,
And the Light shall have the harp of gold.

By the pleasant lake the Sleepers lie,
On Cafdan's Way where the kestrels call;
Though grim from the Grey King shadows fall,
Yet singing the golden harp shall guide
To break their sleep and bid them ride.

When light from the lost land shall return,
Six Sleepers shall ride, six Signs shall burn,
And where the midsummer tree grows tall
By Pendragon's sword the Dark shall fall.

Y maent yr mynyddoedd yn canu, ac y mae'r arglwyddes yn dod.
The mountains are singing, and the Lady comes."

The fourth book, The Grey King, won the John Newbery Medal for 1975. Will Stanton returns, minus the Drew children but with a new ally: Bran Davies, a mysterious albino Welsh boy. Together, they must find a magical gold harp which will wake the Sleepers, ancient warriors crucial to the final battle against the Dark. Opposing them is the Grey King, one of the Dark's most powerful lords. As this story unfolds, so too does that of Bran, the subject of fear and loathing his entire life, not only because of his looks, but because of his mysterious mother, who appeared one day with Merriman Lyon out of the hills, and returned, after leaving Bran with a simple farmer, just as mysteriously. Bran's realization of his awesome power and responsibility is critical to the Light's victory.

The fifth and final book of the series is Silver on The Tree. The Drews, Will, Bran, and Merriman Lyon are joined by a large cast of friends and foes as the final battle between the Light and Dark takes shape. To defeat the Dark, Bran, heir to the Pendragon, must recover Eirias, a magical sword commissioned by his father. The sword must then be wielded to cut the silver blossom from the midsummer tree. Whichever side obtains the blossom will win the war, and thus help decide the fate of humanity.