The whispering knights

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'I don't know that you have done anything wrong,' Miss Hepplewhite said. 'But it is possible that you have done something rather dangerous.' William and Susie thought they were just playing a game when they cooked a witch's brew in the old barn and said a spell over it, but Martha was not so sure. And indeed, the three friends soon learn that they have called up something dark and evil out of the distant past . . .