The Grey King by Susan Cooper (The Dark is Rising #4)
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With the final battle between the Light and the Dark soon approaching, Will sets out on a quest to call for aid. Hidden within the Welsh hills is a magical harp that he must use to wake the Sleepers - six noble riders who have slept for centuries. But an illness has robbed Will of nearly all his knowledge of the Old Ones, and he is left only with a broken riddle to guide him in his task. As Will travels blindly through the hills, his journey will bring him face-to-face with the most powerful Lord of the Dark - the Grey King. The King holds the harp and Sleepers within his lands, and there has yet to be a force strong enough to tear them from his grasp...