Some people can only be pushed so far. Luna will take a lot, but it's best not to insult her mother.
A Harry Potter / Cthulhu Mythos (specifically The Shadow Over Innsmouth) crossover Fanfic by PixelWriter1.
As everyone at Hogwarts knows, Luna Lovegood is frequently bullied for her oddities. She can deal with that. What she won't forgive, however, is Professor Snape insulting her mother a few too many times and calling her a fool for believing in Wrackspurts. Her method of proving him wrong will ensure that nobody ever questions her on that particular subject again (and mentally scar quite a few people in the process).
It's Always The Quiet Ones contains examples of:
- Adaptational Jerkass: Marietta Edgecombe was The Stool Pigeon in the books who told Umbridge about the existence of Dumbledore's Army, albeit with the possible justification that she was afraid of endangering her mother's job at the Ministry, but not an outright bully. In this story, she steals Luna's pendant along with another girl and accuses her of being a liar when asked to give it back.
- Admiring the Abomination: Luna thinks the Wrackspurts are kind of cute, when they terrify the living daylights out of everyone else, to put it mildly.
- Alliance with an Abomination: After letting Cthulhu eat Snape, Luna convinces him to leave everyone else alone and only go after Death Eaters because they taste better.
- Berserk Button: The two girls stealing her mother's pendant and then Professor Snape insulting her mother is what finally drives Luna to prove that Wrackspurts are real...and then things start getting ugly. Like, really ugly.
- Beware the Silly Ones: Luna the lovable Cloud Cuckoolander turns out to be capable of summoning mind-destroying, human-eating alien jellyfish.
- Black Speech: Luna performs a terrifying chant in an an alien language that makes the Wrackspurts visible to everyone.
- Bring My Brown Pants: Snape has a passing thought about events shrinking his bladder. Flitwick is a bit embarrassed to note that his robes are quite damp when matters conclude, then notices he's far from the only one with that problem. He still hurries to change clothes right afterward, though.
- Brown Note: Merely being exposed to the alien dimension for a few minutes is enough to deeply traumatize everyone in the Great Hall who isn't Luna. Numerous patients end up checking into St. Mungo's Hospital, developing incurable phobias of tentacles and fishy smells.
- Bullying a Dragon: Snape tries to attack the Wrackspurts with Stunners even when it's clear that conventional spells can't hurt them. His attempt to use the Killing Curse against Cthulhu only makes it angry and results in him getting eaten.
- Eldritch Abomination: Turns out Wrackspurts are very real, and they're actually mind-rending jellyfish-like horrors from another dimension.
- Eldritch Location: Luna's family spell temporarily lets everyone view an alien dimension filled with horrible fishy things and what is implied to be the Deep One city of Y'ha-nthlei.The illusion on the ceiling faded steadily, but revealed when it finally vanished was not the stonework of the castle, but a view of a sky that belonged to nowhere on earth, pallid stars forming unrecognisable constellations, past clouds radiating unclean light, reflecting the output of some hideous sun hidden beyond the walls. Somehow, they could all tell it was no illusion, no synthetic reality due to clever spells and cantrips, it was a true view of a scene no human should gaze upon. As the words continued, the walls between the columns around the room were, one by one, replaced with the same view, affording all the ability to look out on a glutinous sea lit by that same dying star low on the horizon. Things were in the water, not visible directly, but the slow rolling waves that moved with definite intent through the glistening viscous fluid betrayed their presence.
- Humanoid Abomination: Luna is descended from the Deep Ones through her mother, and capable of communicating with alien horrors that make everyone else quake with mortal terror.
- Humans Are Insects: The second Luna's prayer to the Great Old Ones begins, everyone instinctively feels "something running cold fingers through their soul, for want of a better description, something to which humanity was a mere aberration, irrelevant in the grand scheme of the universe."
- I Need a Freaking Drink: After the Cthulhu incident is safely past, Professor Flitwick considers assigning a three-foot-long essay on why Great Old Ones shouldn't be summoned at dinnertime, or at any time ever, but instead goes out and gets very drunk.
- Karmic Death: After repeatedly threatening and insulting Luna, Snape gets carried away and presumably eaten by her "friend", none other than Cthulhu himself.
- Mugging the Monster: Suffice to say, Marietta and her fellow bully would never have picked on Luna if they knew she could summon Eldritch Abominations from the deep with her mother's family spell. Snape might have stood down on his bullying of Luna, too.
- Narrative Profanity Filter: While ensnared in the fishy tentacles of Cthulhu, Snape shoots a bunch of spells unsuccessfully at his captor, and then unleashes the Killing Curse, which is described as "two words that resulted in a virulent green streak of magic leaving his wand and passing through the alien creature".
- No-Sell: Cthulhu and the Wrackspurts are alien abominations that can't be hurt by magic spells, which pass straight through them. Even the Killing Curse, instantly fatal to a human when properly cast, is merely an annoyance to them.
- Precision F-Strike: Done by Flitwick when the pieces fall into place for him and he realizes just what's about to happen.
- Reasonable Authority Figure: When Luna accuses two girls of stealing her mother's pendant, Professor Flitwick scolds them and Professor Snape for insulting her.
- Screw This, I'm Outta Here: The second Luna's spell begins, every sentient non-human creature inside and outside Hogwarts—including house-elves, centaurs, merfolk and the giant squid—senses it and rushes to get as far away from the castle as they possibly can.
