Valley of Headless Men

There is an eerie, mist covered valley in the jungles of South America. The local Indians call it the Valley of the Headless Men, for many have entered in search of the legendary entrance to the Underworld that is said to be within. And all that do enter, do not return. Search parties are sent, and they do return, with reports that the bodies of the adventurers have been found... decapitated. Time after time, decapitated bodies are found of those who enter in search of the entrance. Those who enter the Valley and are not in search of the Underworld, such as search parties and hunters, always return intact. It seems that those who search, find, but there is something guarding the entrance. Clearly, this entity that guards is eternal, and supernatural.