The Devil's Promise — David Stuart Davies

Sherlock Holmes and Doctor Watson are holidaying on the English coast when they discover a corpse on the beach… which then disappears. They can get little help from the nearby village, populated by strange and unfriendly characters. Then the corpse suddenly reappears in their cottage and they are attacked by persons unknown. Watson comes to, and discovers that months have passed, and Holmes is not the man he remembers. What has happened to his friend? Does it have something to do with a dead devil worshipper, whose children happen to live in the cursed village?