The Grey King: The Dark is Rising by Susan Cooper

Title: The Grey King: The Dark is Rising #4
Author: Susan Cooper

Illustrator: Michael Heslop
Publication: Atheneum, hardcover, 1975
Genre: Juvenile fantasy/series
Setting: Wales

Description: After an unexpectedly severe illness, Will Stanton is sent to Wales to recover with his uncle's family. Approaching their farm, Will unexpectedly feels a sense of menace coming from the mountains and learns there is a legend that the Grey King dwells there.