The Grey King (The Dark Is Rising Sequence, Book Four)

Blurb

The Grey King is a contemporary fantasy novel by Susan Cooper, published almost simultaneously by Chatto & Windus and Atheneum in 1975. It is the fourth of five books in her Arthurian fantasy series The Dark is Rising.
The Grey King won the inaugural Tir na n-Og Award from the Welsh Books Council as the year's best English-language children's book with an "authentic Welsh background". It is set in Wales and incorporates Welsh folklore as well as Arthurian material. It also won the annual Newbery Medal recognising the year's "most distinguished contribution to American children's literature".