The Darzoids' Stone — Richard Smith

A special stone hidden in a quiet cove in Cornwall more than 200 years ago, is the object of a desperate search by two opposing factions. When Harry and his school friends discover a code whilst holidaying in the cove, they assume it will lead them to hidden treasure. But dark forces, with seemingly supernatural powers, are working against them. The teenagers soon find themselves caught up in a deadly battle, and will be lucky to escape with their lives, let alone get their hands on the treasure.