Oxford bookworms library: level 4:: the whispering knights (Oxford University Press)

'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...