"Good afternoon, my young apprentices," said Professor Quirrell. His voice seemed to come from the desk screen and to be speaking directly to Harry. "Welcome to your first lesson in Battle Magic, as the founders of Hogwarts would have put it; or, as it happens to be called in the late twentieth century, Defence Against the Dark Arts."
There was a certain amount of frantic scrabbling as students, taken by surprise, reached for their parchment or notebooks.
"No," Professor Quirrell said. "Don't bother writing down what this subject was once called. No such pointless question will count toward your marks in any of my lessons. That is a promise."
Many students sat straight up at that, looking rather shocked.
Professor Quirrell was smiling thinly. "Those of you who have wasted time by reading your useless first-year Defence textbooks -"
Someone made a choking sound. Harry wondered if it was Hermione.
"- may have gotten the impression that although this subject is called Defence Against the Dark Arts, it is actually about how to defend against Nightmare Butterflies, which cause mildly bad dreams, or Acid Slugs, which can dissolve all the way through a two-inch wooden beam given most of a day."
Professor Quirrell stood up, shoving his chair back from the desk. The screen on Harry's desk followed his every move. Professor Quirrell strode towards the front of the classroom, and bellowed:
"The Hungarian Horntail is taller than a dozen men! It breathes fire so quickly and so accurately that it can melt a Snitch in midflight! One Killing Curse will bring it down!"
There were gasps from the students.
"The Mountain Troll is more dangerous than the Hungarian Horntail! It is strong enough to bite through steel! Its hide is resistant enough to withstand Stunning Hexes and Cutting Charms! Its sense of smell is so acute that it can tell from afar whether its prey is part of a pack, or alone and vulnerable! Most fearsome of all, the troll is unique among magical creatures in continuously maintaining a form of Transfiguration on itself - it is always transforming into its own body. If you somehow succeed in ripping off its arm it will grow another within seconds! Fire and acid will produce scar tissue which can temporarily confuse a troll's regenerative powers - for an hour or two! They are smart enough to use clubs as tools! The mountain troll is the third most perfect killing machine in all Nature! One Killing Curse will bring it down."
The students were looking rather shocked.
Professor Quirrell was smiling rather grimly. "Your sad excuse for a third-year Defence textbook will suggest to you that you expose the mountain troll to sunlight, which will freeze it in place. This, my young apprentices, is the sort of useless knowledge you will never find in my lessons. You do not encounter mountain trolls in open daylight! The idea that you should use sunlight to stop them is the result of foolish textbook authors trying to show off their mastery of minutia at the expense of practicality. Just because there is a ridiculously obscure way of dealing with mountain trolls does not mean you should actually try to use it! The Killing Curse is unblockable, unstoppable, and works every single time on anything with a brain. If, as an adult wizard, you find yourself incapable of using the Killing Curse, then you can simply Apparate away! Likewise if you are facing the second most perfect killing machine, a Dementor. You just Apparate away!"
"Unless, of course," Professor Quirrell said, his voice now lower and harder, "you are under the influence of an anti-Apparition jinx. No, there is exactly one monster which can threaten you once you are fully grown. The single most dangerous monster in all the world, so dangerous that nothing else comes close. The Dark Wizard. That is the only thing that will still be able to threaten you."
Professor Quirrell's lips were set in a thin line. "I will reluctantly teach you enough trivia for a passing mark on the Ministry-mandated portions of your first-year finals. Since your exact mark on these sections will make no difference to your future life, anyone who wants more than a passing mark is welcome to waste their own time studying our pathetic excuse for a textbook. The title of this subject is not Defence Against Minor Pests. You are here to learn how to defend yourselves against the Dark Arts. Which means, let us be very clear on this, defending yourselves against Dark Wizards. People with wands who want to hurt you and who will likely succeed in doing so unless you hurt them first! There is no defence without offence! There is no defence without fighting! This reality is deemed too harsh for eleven-year-olds by the fat, overpaid, Auror-guarded politicians who mandated your curriculum. To the abyss with those fools! You are here for the subject that has been taught at Hogwarts for eight hundred years! Welcome to your first year of Battle Magic!"