The Arithmancer/Nightmare Fuel


 * In-Universe, the Cthulhu Mythos is literally this for Hermione (and later Ludo Bagman).


 * What Hermione did to Dolores Umbridge in a fury-fueled fugue of rapid-fire spellcasting -- afterwards, Umbridge is described as looking "like a modern art sculpture", but is still alive. And at least one attempt to fix her made it worse.


 * The method by which Hermione defeats Bellatrix Lestrange --.