Which witch? (1979)

Arriman the Awful, feared Great Wizard of the North, has decided to marry, because he must sire a child to carry on his tradition of Loathing Light and Blighting the Beautiful. But his wife must be a witch of the darkest powers... A sorcery competition among the witches of Todcaster is held to discover which witch is the most potent and fiendish. The competition begins, and each witch it turn pulls out of the cauldron her especially wicked spell. But some go terribly, horribly wrong. 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?