The Unforgivable Three
(No! Please! Have mercy!
Mercy? Poor deluded mudblood... Your kind do not deserve any mercy. Crucio!
Aaaah!)
Pain. Agony. Suffering. Those words cannot describe the whole reality that result from casting the Pain spell on a living creature. The feelings are so intense that they can and will break the will of anyone if the curse is maintained for long enough. And so, without a will, the mind can be easily placed under certain influences...
Imperio!
Absolute control. With one word you can command the weak willed to do anything. Impossible to detect, a spell than can turn you worst enemy into your most loyal slave. If the weak willed are vulnerable, then what wold happen to someone with no will at all?
(Now, you will do exactly as I say...)
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(As Dumbledore began his yearly speech in the Great Hall of Hogwarts, students with dubious magical lineage stood up from the three House tables, pointing their wands toward the director and uttered two terrible words...)
Avada Kedavra!
Two words, a green flash and absolutely no defence. Really, only a madmen or an idiot wouldn't abuse the hell out of it.
The Hogwarts professors always said that Tom Riddle was a genius...