Cassian straightened from the cauldron, hair was doing several things at once, sticking up, curling in odd places, and somehow glinting with something that had no business being luminescent. A pair of thick Muggle safety goggles perched on his forehead, one lens smudged with soot, the other slightly fogged from whatever steam had last belched out of the cauldron. The lab coat hung over his robes, streaked with green stains and what might’ve been goo or burned basilisk scale. Maybe both. No one wanted to ask.

Bright yellow rubber gloves, standard dishwashing fare, one finger half-melted. His boots were charmed against spills but still had something bubbling faintly on one sole. He looked, frankly, like the neighbourhood’s friendly mad scientist, the kind you didn’t mind living down the road from, as long as the smoke stayed on his side of the fence.

“We’ve finally got it,” he said, wiping a smear of something unidentifiable off his cheek with the back of his wrist.

Bathsheda, on the other hand, was all clean lines and charm-bound calm. Just robes and a wand. Magic layered over her like an invisible apron.

“Seems like it,” she nodded, giving the mix a faint stir. The stench wafted up like something had died twice. “To think this is how I’d find out the one smell I’m most nauseated by.”

Cassian peeled off the glove and let it flop wetly to the bench. “I feel like I’ve been inhaling old feet for an hour.”

“You have.”

“Brilliant.”

He poked the side of the cauldron with his wand. It hissed in response, thick sludge bubbling. The surface shimmered, then spat out a violet spark that hit the floor and tried to crawl away.

Bathsheda pointed her wand at it. “Don’t.”

The spark froze. Sizzled. Died.

Cassian cleaned himself off with a wave of his wand. His hair deflated slightly, his face returned to its usual colour, and the glove residue vanished from his fingers.

“We’ll fix that part later,” he muttered, eyeing the cauldron. Then he grinned. “For now, cheers, we’ve found a detection-potion for Love Potion.”

Bathsheda smiled too, though she didn’t lean in. The smell was still foul.

They’d been at it for years, tinkering with counteragents, building resistance chains, arguing with potion theory that hadn’t been updated since someone decided Amortentia didn’t need to be regulated because “true love conquers all.” The Ministry never banned it, just slapped a label on it and let shops peddle it like romantic chewing gum. So Cassian and Bathsheda had decided to handle it themselves.

Cassian had spent months teaching everything short of making students smell heartbreak in a bottle. Between the history, the theory, and a handful of very real antidote drills, he’d run more Amortentia lessons than most potion masters managed in a career. The more he taught, the sharper he got, his mastery shot up fast. By the time they started testing reversals, Cassian could break down Amortentia’s structure in his sleep and spot ingredients’ scent from a corridor away.

“To think it turns Amortentia’s side-effect, the smell you love most, into the thing that makes you sick…” Cassian wrinkled his nose. “Fascinating. Horrible. But fascinating.” He clamped a hand over his nose. “Mine smells like burnt treacle, damp parchment, and… whatever that potion cupboard was hiding last term. I’m offended on a personal level.”

It was the exact bouquet of everything he avoided in life, scorched sugar, old books left in a cellar, and that odd medicinal tang from a classroom he definitely did not blow up.

Bathsheda nodded, fanning herself slightly. “Smells like stale ink and warm cabbage in here. I think my grandmother’s wardrobe just came back to haunt me.”

He tilted the cauldron slightly, watching the mixture settle. “That’s good, actually. Strong aversion response. It means it’s working.”

“Or it means I’m going to be sick in your boots.”

Cassian gave her a look. “You’re a grown woman. Aim for the sink.”

The potion burped again, this time spitting out something that looked suspiciously like a half-dissolved heart. It sizzled on the stone with a fizz.

Cassian pointed at it. “Romance.”

Bathsheda didn’t answer. She was already jotting down notes, lips pressed tight.

He leaned over her shoulder.

“So. We file the paperwork?” She asked.

Cassian grinned. “Of course.”

So the Rose Bath was born.

Sounded delicate. Promised the smell of a dream. The product, however, could knock a troll flat.

When the name first started drifting through the Prophet, people thought it was a new perfume line. Something high-end. French, probably. Definitely expensive. The kind of thing you’d gift your mistress if you were a wizard with questionable morals and too much gold.

Nobody expected it to be a Love Potion detection. Least of all one developed by Hogwarts staff.

Cassian and Bathsheda didn’t rush it. They let the rumour ferment. Paid a few gossips to murmur in the right corners, dropped exactly one anonymous ad into Witch Weekly, nothing but a rose petal and the words “Smell the truth.”

By the time the week before Valentine’s rolled round, anticipation had built into a small frenzy.

During dinner, the two of them rose at the High Table.

The hall quieted in seconds. Students of Hogwarts, five schools, guests, professors, everyone turned to stare.

Cassian offered a charming smile, hands tucked neatly into his pockets. “Evening. If you’re wondering why your lovely Ancient Runes professor and I have decided to interrupt your meal, no, we’re not eloping. But we do have an announcement to make.”

He held up a small glass vial. Pale rose‑coloured.

“That, my dear students and traumatised faculty, is the first legally approved, Ministry-acknowledged, absolutely horrid-smelling Love Potion detector ever brewed.”

The silence following was so loud, Cassian winced, like someone had dropped a fork on a plate in the middle of a funeral. Then came the uproar, whispers firing off across the hall, cheers rising from a few brave corners, some full-on gasps.

The staff table wasn’t much better.

Cassian gave the room a sunny smile. “Application’s delightfully simple. Just a drop into a glass of water, and voila, if anything nearby’s spiked with Love Potion, it reacts. Reverses the side effect.”

Maxime looked horrified. “Which side effect?”

“Smell,” Bathsheda said, already grinning.

Cassian nodded. “Turns it right on its head. Whatever the potion was pretending to smell like, it’ll hit you with the scent of rotting plums, or your nan’s favourite socks. Works immediately, and no, before you ask, it doesn’t explode. We tested that.”

Snape got up so sharply his chair scraped. “I’ll be the judge of that.”

He sounded personally insulted any time someone invented a potion without consulting him first.

With a flick of his wand, a bottle shot off somewhere below the castle and landed neatly in his hand. He popped the stopper, and the effect was immediate, half the hall inhaled like they’d wandered into their favourite bakery, or meadow, or perfume counter. Whatever the potion wanted them to smell, it delivered.

Cassian, grinning like he’d been waiting for this exact moment, held out the vial of Rose Bath. “Go on, Severus. Let’s ruin some evenings.”

Snape did not rise to the bait. He snatched the vial, tipped a drop into a glass of water, and the room cracked with a sharp little sound, like ice breaking.

The reaction hit the hall at once.

Every student who’d been mid‑blissful inhale gagged. Some doubled over. A Hufflepuff slapped a hand over her mouth. Someone in Ravenclaw choked out, “Merlin’s socks, what is that?”

The scent washed through the hall in a wave, rot, cabbage stew, heartbreak, whatever each person loathed most. Nobody escaped. Even Professors recoiled.

Cassian watched it all with a wide smile, utterly delighted. “There it is.”

Snape stood stiffly over the glass. His face didn’t move, but the tiniest twitch in his jaw suggested he’d caught a whiff of whatever personal hell the antidote had picked for him.

Bathsheda cleared her throat. “As you can see, the counter‑potion works. Instantly.”

Pomfrey had a hand over her nose. “Sweet Circe, warn a woman next time.”

Cassian shrugged. “If you smelled roses, you’d accuse us of lying.”

Around the hall, students were still fighting the lingering stink.

Cassian’s grin dropped off his face. He looked over the hall, deadpan.

“I hate that people need a potion for this,” he said, way too calm. “In a decent world, none of this would’ve been needed. But here we are.”

A few students shifted, trying not to make eye contact with him. Others sat frozen, hands clamped over their noses.

“This’ll be sold at cost,” Cassian went on. “No profit. Not a Knut. If anything, I’ll end up shoving boxes of it into schools for free. Shops, too, once we get the supply line sorted.”

Bathsheda folded her arms, watching the students.

“Professor Babbling and I pushed for this because Love Potions are sick,” Cassian said. “People who use them are worse.”

The hall had gone quiet.

“Now that there’s a way to catch it before it hits your drink,” Cassian continued, “let me be painfully clear… If anyone here is caught with the stuff, using it, brewing it outside of the classes, slipping it into so much as pumpkin juice, being expelled will be the least of your problems.”

Someone near the Slytherin table let out a tiny, fearful breath. Cassian heard it.

He leaned forward slightly. “Try me.”

The room didn’t breathe.

He straightened, dusted his hands on his robes like he’d finished wiping down a chalkboard, and looked to Bathsheda. She gave a nod.

Cassian turned back to the hall. “Right. Enjoy your dinner.”

And just like that, noise rushed back into the hall, whispers, groans, students airing out collars like they were escaping a gas leak.

He flicked his wrist, and the smell in the hall finally began to fade.

As they headed back toward their seats, Cassian muttered, “Think they took that well.”

Bathsheda didn’t look up. “I think half of them might sleep with the lights on.”

“Good. Keeps them honest.”

Then everything went to hell in a handbasket.

A sharp crack of glass rang out. Heads turned. The Hufflepuff table jolted, someone yelped. Cassian’s gaze snapped to the far end of the Beauxbatons section.

Fleur was on her feet.

Her plate was shattered, bits of glass glittering at her boots. Her expression wasn’t angry. It was worse. Disgust, plain and unfiltered, twisted across her face like she’d stepped in something vile.

Cassian froze mid-step. Bathsheda, already half-turning to sit, turned back, both staring.

“Oh,” Cassian said slowly. “Is the smell too much? Are there Veela side effects I should know about?”

His eyes flicked to Gabrielle, expecting maybe a matching reaction. Nothing. The girl looked fine. Calm. Disgusted. Confused, but fine.

Bathsheda’s brow creased. “She’s not reacting to the smell.”

Cassian followed Fleur’s line of sight.

She wasn’t looking at the bottle. Or the High Table.

She was looking straight at Mingyu.

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