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Hermione Granger, Muggleborn witch, found her way to Diagon Alley within a week after McGonagall left. She introduced herself with all necessary precociousness to Ollivander, got a wand, and proceeded to explore this amazing new world in which she found herself.
And she found a book. It appeared to be a children’s book. The Tales of Beedle the Bard. She thought it would be useful to know what her schoolmates would have as literature. After all, she supposed they hadn’t read Beatrix Potter’s works, nor Lewis Carroll, had they? Would the world of magic be acquainted with Tolkien? Narnia?
In this book she found reference of Unforgivable Curses. She practiced saying them in her head, but not aloud, for they were Unforgivable.
But amongst the Wizarding people, and on the train, she felt compelled to prove herself - to show that yes, she did belong.
“I read Beedle the Bard,” she remarked casually.
“Oh?” the bespectacled boy across from her asked. “Were there spells in there? I’ve only just got my wand. Do they work?”
And … the rest is history.
Due to her age and utter ignorance of the Magical world, Hermione Granger was not sent to Azkaban, but her magic was bound and she and her parents Obliviated.
This is, according to Hogwarts, a History, the only time a Muggleborn ever tried an Unforgivable before taking even one class at Hogwarts.
Harry Potter, who survived the curse—again—found that the scar on his forehead was much paler than it had been, and when this was (years later) reported to Headmaster Albus Dumbledore, Hermione Granger was considered something of a minor hero in the Second Blood Purity War.
Footnote: Hermione Granger, as of the publication of this edition of Hogwarts, a History, is still ignorant of the Wizarding world. She is a barrister in England, known for her reasoned legal opinions.