Which Witch? — Eva Ibbotson

WANTED:
ONE WILDLY WICKED WITCH TO WED.

Arriman the Awful, the handsome Wizard of the North, has decided to marry. But which witch will it be? Arriman devises a contest: whoever performs the darkest piece of magic will become his bride. Belladonna, the smallest of the coven, wants desperately to win, but she has one problem: she's a good Witch. Her spells conjure up begonias and baby birds, not a single nest of vipers or bloodshot eyeballs. But Belladonna is in love, and she's determined to do something sinister in time for the contest....