The Leviathan by Rosie Andrews

Norfolk, 1643. Reluctant soldier Thomas Treadwater has been summoned home by his young sister in a letter accusing their new servant of improper conduct with their widowed father.

By the time Thomas reaches the family farm, his father is on the verge of death, Esther is near hysterical and their new servant is in prison, facing charges of witchcraft. Thomas prides himself on being a rational, modern man. He is confident that he can free their servant, a beautiful if peculiarly self possessed young woman, and reassure his sister that there is nothing further to fear, now he has returned.

But as he begins to unravels the mystery of what has happened to his family, he uncovers a tale, not of witchcraft, but of something dark and ancient, linked to a shipwreck many years before... Something has awoken, and now it will not rest.