Professor Eudice Eusyram is a Harry Potter fanfiction published on FictionAlley by author I Smell Rates on April 7, 2005. The work is principally a satire of "Mary Sue" fanfiction and other fanfiction clichés. In fact, the last name of the title character is "Mary Sue" spelled backwards.
The work is set Harry and his friends' sixth year at Hogwarts and begins with the introduction of Eudice Eusyram, the "perfect" Defense Against the Dark Arts teacher, who quickly wins over all the Gryffindor students. Severus Snape, however, becomes jealous of her and plots to get back at her.
To do this he puts "Phreneticus Potion," a potion which "impairs the drinker's self-control and judgment," in Eudice's pumpkin juice and, deciding to "have some more fun," he puts some of it Harry, Ron and Hermione's cups as well.
Eudice, however, immediately realizes what he's done and doesn't drink hers, but doesn't stop Harry and his friends from drinking theirs knowing it would be "the only why everyone would end up together the way they were supposed to."
Once the main characters are paired off as they were always "meant" to be, Snape, jealous again, challenges Eudice to duel. She accepts and, in fact, fights him in the nude and wins. He is forced to drink the Phreneticus Potion himself and his resulting sex with Neville Longbottom causes him to be so embarrassed that he moves to Mexico.