Because sometimes, when there's more than one reality at play, too much dreaming can make the walls between them come tumbling down. Unfortunately there's usually a damned good reason for there being walls between them in the first place - to keep things out. Things who want to make mischief and play havoc with the natural order. Granny Weatherwax and her tiny coven are up against real elves. And even in a world of dwarfs, wizards, trolls, Morris dancers andthe odd orang-utan, this is going to cause real trouble. With lots of hey-nonny-nonny and blood all over the place.