In a remote Himalayan cave a group of New Age tourists come across the mummified corpse of an RAF flyer. Before they can investigate the mystery, all except the guide are ritually massacred. In Bosnia, an American Air Cavalry patrol investigates a disturbance at a mass grave site and its commander catches a fleeting glimpse of a creature straight out of a medieval morality play. We are on the verge of discovering a new frontier. And it lies beneath our feet. A global labyrinth of underground tunnels and caves, miles below the surface, inhabited by immensely strong, savage devil-like creatures. Hell exists. The world's armies respond to the threat and are slaughtered in the dark. But from the panic comes a new resolve...