"The Witchcraft of Mary-Marie" is a short story by L. Frank Baum, author of The Wonderful Wizard of Oz. It was first published in the 1908 Bobbs-Merrill edition of Baum's American Fairy Tales, and was reprinted in the Autumn 1994 edition of The Baum Bugle, with new illustrations by Eric Shanower.
Mary-Marie is a poor girl, who survives by foraging for berries and nuts. Her mother is dead, and her father is missing in the foreign wars of their land's cruel king, the usurper Gruph. One day, an old man convinces the girl to become a witch. Seeking out the local practitioner — an elderly but still beautiful white witch — Mary-Marie becomes her student. In lieu of payment, Mary-Marie agrees to perform three tasks at the end of her training.
That training goes well, and in time the girl becomes adept at magic. Mary-Marie is not happy when she learns the nature of the three tasks; but she accepts her responsibility to fulfill them. Disguised as Princess Pritikin, neighboring royalty, the witch-girl goes to the court of King Gruph. She is welcome there, and finds it easy to transform the king into a billy goat. Back in her own ragged clothes, she leads the bleating goat to the town of Ribdil and the butcher Gurd. Gurd is a big, ferocious, ruthless man; his neighbors shun him. Mary-Marie sells him the goat, but Gurd refuses to pay; when the witch-girl confronts him, Gurd threatens to kill her after he butchers the goat. Mary-Marie realizes that Gurd is a powerful magician, who works as a butcher only because he enjoys it.
Gurd stabs the goat to the heart; the beast suddenly returns to its natural form as King Gruph. Gurd is horrified by what he's done; he himself suddenly turns into a weak little old man. Gurd tells the girl that her mistress has avenged her wrong, and that he will be dead in an hour.
Mary-Marie returns to her teacher, but finds that the old witch has transformed into the missing Prince Melra, Gruph's nephew and the land's rightful king. Melra had been enchanted by Gurd, and managed his plan to reverse the enchantment. (He'd also been the old man who convinced her to study magic.) Melra is in love with Mary-Marie's beauty and sweetness; the girl thinks she's hardly old enough to marry him...but decides that if other girls can marry young, so can she.
The story conforms to Baum's general metaphysic. At first, Mary-Marie believes that all witches are old, ugly, and evil; but her misapprehensions are quickly corrected.