and so , with the macabre urge to begin with `` once upon a time ... '' i leaned back against the wall , and with ian 's head still in my lap , began the story of rose hall and its mistress , the witch geillis duncan ; of the reverend archibald campbell and his strange sister , margaret , of the edinburgh fiend and the fraser prophecy ; and of a night of fire and crocodile 's blood , when the slaves of six plantations along the yallahs river had risen and slain their masters , roused by the houngan ishmael .