Prime evil - Diana G. Gallagher

When attractive, youthful Crystal Gregory takes on a teaching post at Sunnydale High, she also takes what seems to be an instant dislike to Buffy. At the same time she establishes favourites - among them Michael, Anya, and Willow - who are loyal to her in the extreme. This wouldn't bother Buffy - except that she's noticed fresh burn marks on two of the students' necks, which Giles identifies as the symbol of Shugra, a witch who supposedly lived long ago, before the Salem Witch Trials.
The Primal Witch plans to gather a coven of thirteen people each of whom has some magickal ability. With their help she will perform a moon ritual to endow herself with infinite power, after which all people will be subject to her will. Buffy will need her Wicca-girl, Willow, to save the world this time. But can Willow be persuaded by reason to free herself from Crystal's spell - before it's too late?