Greenwitch The Dark is Rising sequence

In their desperate quest to find the grail, stolen by the forces of the Dark, the Drew children have returned to Trewissick. One night, in a strange ceremony, Jane watches the village women construct the Greenwitch: an ancient offering to the sea. Overcome by the Greenwitch's power, Jane makes a perilous wish - one that will tip the balance in the ongoing struggle between good and evil.
The third haunting, magical book in the highly acclaimed Dark is Rising sequence.