While Susan Cooper herself hasn't explicitly stated one single inspiration for the book, several factors likely contributed to its creation:
* Ancient mythologies and folklore: Cooper was deeply interested in mythology and folklore, especially those of the Celtic and British Isles. The "Dark Is Rising" series is heavily influenced by these traditions, drawing from Arthurian legend, Celtic myths, and ancient pagan beliefs.
* The power of stories: Cooper believed in the importance of storytelling and the power of myth to shape our understanding of the world. She aimed to create stories that would resonate with readers and explore themes of good and evil, fate and free will, and the battle between light and darkness.
* Personal experiences: Though not explicitly stated, Cooper's own experiences with loss, grief, and coming of age likely informed the themes of the books. The protagonist, Will Stanton, faces a profound personal journey as he discovers his destiny and confronts the forces of darkness.
While there may not be a single, definitive inspiration, these elements combined to create the rich, evocative world of "The King of Shadows" and the "Dark Is Rising" series as a whole.