Prophecy GeneratorWhether mumbled by a badly-dressed lunatic with crazy hair, or discovered on ancient parchment in a hidden tomb of a long-dead wizard, there's no denying the power of prophecy. A good one can really spice up your fantasy. It should be vague and ambiguous enough to be almost completely meaningless until the very end, when it all comes together to make perfect sense.
If you don't want to go with something as pre-ordained as prophecy, there's the mystical compulsion of the geis, a taboo that could doom a hero if disobeyed. It sealed the fate of the Irish hero Cúchulainn, so it might be just what your story needs.