Swallow Them Up [first edition]

Intrigued by an invitation from a man named Oxley, nine guests arrive at an island castle. But when Oxley informs them that some of them have harmed him in the past and he intends to exact retribution, they decide he must be mad, for none can remember having met him before. Then all links with the mainland are cut, a guest is assaulted, and another disappears. Oxley too goes missing - but the violence continues, culminating in murder. Is there another madman on the island? Is he, or she, one of them?