Silver on the Tree by Susan Cooper is the final book in "The Dark is Rising" sequence. Will returns to Wales to meet Bran, Merriman, and the three Drew children for the final quest before the rising of the Dark. It is their task to recover the crystal sword from the Lost City, the last of the ancient weapons needed by the Light for the final battle. The dark conspires to hold Merriman up, and Will and Bran must travel to the Lost City alone while Merriman protects the Drew children. With the sword in hand, Bran takes his rightful place in the final battle as both Light and Dark strive to reach the magic of the midsummer tree at the appointed time for the one who cuts its flower in the moment of bloom will hold the power to send the other out of time forever.
Another excellent book in this series that I would recommend for ages 11 and up. The book draws the sequence full circle back to the Arthurian legends upon which it began as the Old Ones battle the Dark in the first great battle in the time of Arthur and the last great battle in the present time. These books are going on my Don't Miss list for all of the kids when they are old enough.