Showing Greenwitch by Susan Cooper

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.