In Peter S. Beagle's The Last Unicorn, Schmendrick the Magician improvises these:
I'll make you into a bad poet with dreams! I'll change your heart into green grass, and all you love into a sheep! You pile of stones! I'll set all your toenails growing inward, you mess with me!
But he also said, "The most professional curse ever snarled or croaked or thundered can have no effect on a pure heart."