The Dark is Rising sequence by Susan Cooper

Ever since I first read these books as a child (or teenager? Don't remember!), these lines have been engrained in my memory:

"When the Dark comes rising, six shall turn it back;

Three from the Circle, three from the track;

Wood, bronze, iron; water, fire, stone;

Five shall return, and one go alone. 

Iron for the birthday, bronze carried long;

Wood from the burning, stone out of song;

Fire on the candle ring, water from the thaw;

Six Signs the circle and the grail gone before. 

Fire from the mountains shall find the harp of gold

Played to wake the Sleepers, oldest of the old;

Power from the green witch, lost beneath the sea;

All shall find the light at last, silver on the tree."

The Dark is Rising sequence is a series of five children's books was written by British-born American author Susan Cooper between 1965 and 1977:

• Over Sea, Under Stone (1965), in which the Drew children (Simon, Jane and Barney) go on holiday in Cornwall with their "Great Uncle Merry" and find themselves embarked on a quest to retrieve the Grail before the forces of the "Dark" do.
• The Dark is Rising (1973), in which young Will Stanton discovers on his 11th midwinter birthday that he is the last of the circle of the "Old Ones" and as he gains knowledge and power he must find and unite the six Signs of the "Light" in order to push back the "Dark" when they reach the summit of their power on Twelfth Night (story takes place between Midwinter's Eve and Epiphany). 
• Greenwitch (1974), in which the Drew children find themselves back in Cornwall after someone has stolen the Grail, and join with their Great Uncle Merry and Will Stanton to retrieve it and obtain the missing key element from the temperamental marine "Greenwitch".
• The Grey King (1975), in which Will Stanton finds himself in Wales recuperating from an illness, and makes friends with Bran Davies, a white-haired boy of mysterious origin, with whom he must work to obtain the harp of gold.
• Silver on the Tree (1977), in which things come to a head at the final confrontation between "Light" and "Dark" with the Drew children, Will Stanton and Bran playing key roles.

At first it seems the only link between the first two is the mysterious Merriman Lyon, a key figure in the epic battle between the "Light" and the "Dark". But once the main characters (children) come together in Greenwitch it all becomes clear (that is if you hadn't figured it out yet by the end of The Dark Is Rising).

What have these novels to do with Arthurian lore? They were apparently inspired by Arthurian legends, as well as Norse and Celtic mythology. The grand framework of these tales is the eternal and epic battle between the forces of "Light" and "Dark" which comes to a head at different moments throughout History. In these tales King Arthur was one of the greatest human leaders of the "Light" who helped to stem the onslaught of the "Dark" in Britain for many years, helped by his advisor Merlin, the first of the "Old Ones". After his final battle he was taken away to a place of refuge to heal, to come back for the Final Battle when the "Light" shall need him once again.

Many references are made to Arthurian lore throughout the book, starting in Over Sea, Under Stone with the Drew's arrival in "the land of Logres" on their holiday, and Barney mentioning it is the old name of Cornwall from King Arthur's time (and from that moment on I have always been particularly fond of Barney, and Arthurian fanatic like myself!). There is also of course the children's quest for the Grail (brought to Logres by Bedwin, one of Arthur's last knights), and the hint that their Great Uncle Merry might be more than he appears (Merriman Lyon = Merlyon = Merlin?)...

Then in The Dark is Rising, Will learns of the fight between "Light" and "Dark" and of the many champions the "Light" has had throughout History, without naming the greatest (but it's implied quite heavily):

"the Old Ones spent themselves for three centuries on bringing their land out of the Dark, with the help in the end of their greatest leader, lost in the saving unless one day he might wake and return again."

He is mentioned again by Merriman (again, nameless), when he and Will are looking upon an ancient ship burial that has been unearthed by a flood:

"And one was the greatest ship (burial), of the greatest king of all, and this they have not found and perhaps never will. It lies in peace."

Greenwitch doesn't have any specific Arthurian references, except the presence of the Grail again, as well as Merriman, but The Grey King is steeped deep in Arthurian references, starting with the origins of a mysterious key character (spoiler details at the bottom of this post): the albino boy Bran Davies and his white hound Cafall. And, although we're not officially introduced to him, we do meet Arthur himself during the Deep Magic's test to retrieve the Golden Harp.

"Who was this lord in the sea-blue robe, with his close interest in Bran?"

But it is in the final volume, Silver on the Tree, that it is finally clear that Arthur is an intrinsical part of these tales and is mentioned on more than one occasion (as well as putting in an appearance). Bran refers to him in order to defeat the Afanc in the Bearded Lake when it goes after   Jane. And we have a brief visit with Arthur at Badon (supposedly the previous "major" fight against the "Dark", which gave the "Light" victory for only a short time):

"It is Mons Badonicus, the Battle of Badon, where the Dark comes rising... 

(...) a boat longer and larger than their own. (...) But there was the bearing of a king in the figure at its prow: a square-shouldered man with sunburned face and clear blue eyes; brown hair streaked with grey, and a short grey beard.  

(...) "We go well my lion. We have them, now, at last; they will go back to their own lairs and settle, and leave us to live in peace. For a while...""

and on the ship as our heroes head for the tree in the last effort to defeat the "Dark" forever. Bittersweet moments of offer of another life, joy of acceptance, sadness and pride in a farewell.

"I belong here."

And as the battle between "Light" and "Dark" comes to an end, it is made clear (in a speech that I absolutely love!) that Arthur's task is finally completed as well and he will not return as Merriman tells the children:

"For remember, that it is altogether your world now. You and all the rest. We have delivered you from evil, but the evil that is inside men is at the last a matter for men to control. The responsibility and the hope and the promise are in your hands - your hands and the hands of the children of all men on this earth. The future cannot blame the past. The hope is always there, always alive, but only your fierce caring can fan it into a fire to warm the world. 

For Drake is no longer in his hammock, children, nor is Arthur somewhere sleeping, and you may not lie idly expecting the second coming of anybody now, because the world is yours and it is up to you. Now especially since man has the strength to destroy this world, it is the responsibility of man to keep it alive, in all its beauty and marvelous joy. 

And the world will still be imperfect, because men are imperfect. Good men will still be killed by bad, or sometimes by other good men, and there will still be pain and disease and famine, anger and hate. But if you work and care and are watchful (...) then in the long run the worse will never, ever, triumph over the better."

Elements from Arthurian Legend

• Merriman Lyon a.k.a. Merlin! Here first of the "Old Ones", immortal defenders of the "Light", and somehow a kind of adopted Great Uncle to the Drew children ("Gumerry").
• King Arthur a.k.a. the Pendragon, Britain's greatest hero who according to legend is slumbering in Avalon (or somewhere) and will return to in Britain's hour of need (or in these tales for the final confrontation between "Light" and "Dark").
• Guinevere / Gwen, Arthur's wife who once "betrayed" her lord (clear reference to the Lancelot affair)
• The Grail (and the Quest for the Grail). Not quite the same here, as it isn't the Cup used by Christ and his disciples at the Last Supper (the basis in the original Arthurian tales), instead it is a thing of Power created for the "Light", with images depicting Arthur's great victories and instructions to help the "Light" in its battle against the "Dark".
• Cafall is present in some versions of the legend as Arthur's white hound. Here Bran's adopted father Owen Davies gave him the pup and named it Cafall, as deep inside he knew the truth of his son's origin. 

"Yes, indeed. My father had a dog named Cafall."

• Badon - Mons Badonicus, Arthur's great victory against the Saxons, that ensured peace in Britain for a generation.
• The fact that Arthur does come again for the final battle between good and evil, fulfilling the Arthurian legend that he does but sleep somewhere and will return when Britain needs him.

Camelot with a little twist (spoilers!)

Bran Davies a.k.a. the Pendragon is Arthur and Guinevere's son! When pregnant Guinevere feared Arthur wouldn't believe the child was his (this is after the Lancelot issue) and so she asks Merlin to take her to another place in time where the child will be born and raised in safety. It is Merlin who conveniently decides at what point in time (which will best serve the purpose of the "Light"). Gwen leaves Bran with his "father" Owen Davies and returns through time to Arthur. And when Will finally puzzles it out...

"He saw now that he, Will Stanton, last of the Old Ones, had been fated all along to aid and support Bran in the time to come, just as Merman had always been at the side of Bran's great father. The father who had not known of his son's existence, back when he had been born, and who only now, over he centuries, as a Lord of the High Magic seen him for the first time!"