The Prisoner by Cade Merrill
Cade Merrill is the cousin of Heather Donahue, one of the three students who disappeared when they went into Maryland's Black Hills to research a documentary on the legend of the Blair Witch. Searchers found only their camp and some film footage which became 'The Blair Witch Project', the cult box-office success of Summer 1999.
Cade subsequently collected further case files, in the hope of finding the person who holds the key to the mystery of the Blair Witch. Each 'file' narrates an individual case, with occasional inserted observations noting Cade's own research and verification of the facts.
At 18, Eliza Baynes was convicted of murder. She claimed that the real killer was under the control of the Blair Witch. Now she wants Cade's help. But, she warns, if I tell you my story, you will die . . .