Greenwitch / Susan Cooper.

Enhanced descriptions from Syndetics:

The Drew siblings must face a powerful creature from the ocean depths to reclaim the golden grail in this third installment of Susan Cooper's epic and award-winning The Dark Is Rising Sequence, now with a brand-new look!

The priceless golden grail that Simon, Jane, and Barney Drew worked so hard to recover has been stolen by forces of evil. Great-Uncle Merry takes the siblings back to Trewissick in Cornwall, where he expects the Dark has hidden the grail. There, they are joined by Will Stanton, a mysterious boy with astounding powers.

But there are more forces at play than they realize, and when the village women create the disturbing ritual creature called the Greenwitch--an ancient image made of leaves and branches and cast into the sea for good luck in fishing--Jane must face the unknown without help. The Greenwitch springs to life with vengeful power and is called forth from the ocean depths by the Dark to set loose the unpredictable Wild Magic of the earth. To prevent this dreadful entity from unleashing devastating damage, Jane must convince Greenwitch to turn toward the Light. But can she ever hope to tip the balance against the Dark?