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 -- having her eaten alive by magical nanites/von Neumann machines completely under Hermione's control, until even her bones are turned to ash.