Which Witch?

Arriman the Awful, feared Wizard of the North, has decided to marry. But his wife must be a witch of the darkest powers...
A sorcery competition is held to discover which witch is the most potent and fiendish, and glamorous Madame Olympia conjures up a thousand plague-bearing rats! Belladonna, the white witch, desperately wants to be a wicked enchantress, but her magic produces flowers instead of snakes. How can she become more devilish than all the other witches?
`Eva Ibbotson weaves a magic like no other. Once enchanted, always enchanted.` Michael Morpurgo