The potion.
When Arundel entered the dungeons a door slammed and somebody screamed. She immediately thought about the Grants Artefacts until a student ran past her with her hair on fire. "PUT IT OUT," the girl screamed. Arundel flicked her hand down through the sky and water followed, it drenched the screaming girl who stood with her mouth open blinking and making a strange eww noise.
"What happened?"
"Snape happened." the girl pouted, her face going red. "He made me test my own potion and he knew I'd done it wrong."
"Hay," Arundel shrugged, "you created a potion to make somebody's hair set on fire. Congratulations. Never be so rude about a professor again, five points from Ravenclaw." She'd always wanted to do that.
"Wait...what?"
Arundel smiled.
"You're a professor?"
"Oh yes, Care of Magical Creatures."
"Why are you down here?"
"Why are you so interested?"
"I'm-" the girl paused and at that moment caught Arundel's eye. Arundel saw her snooping, saw her friend being pinned to the ceiling by ghostly hands and having to use a curse to cut him down. She saw them fighting with a Slytherin who they thought was involved. All in the blink of an eye.
"I'm…?" Arundel pressed.
"My name is Freya Griffin, I'm a sixth year."
"Freya, it's not sensible to be out of class alone in the dungeons. Anybody would think you set your hair on fire on purpose to get out of class."
Freya took a step back, but with the practised ease of a school child, she shook her head, eyes wide. "Honestly professor, I just needed to get to the bathroom."
"And now you don't. I'll walk you back."
Never trust a child who says 'Honestly.' with satisfaction she saw Freya sag as she turned and was lead back to potions room.
Snape looked up as the door opened, his eyes studied Freya, then fixed on Arundel. The same old mix of anger and resentment that masked his many insecurities so well. Arundel smiled at him, he did make her laugh. "I've brought you a student, Professor."
"Miss Griffin thinks it's acceptable to run away from her mistakes, I think five points will be sufficient for it."
"But!"
"Be silent Griffin," Snape snapped. "Take your seat and fix your useless potion."
Arundel crossed the classroom and whispered in Snape's ear, "May I borrow a caldron and some supplies?"
"Are you going to pay for what you use?" Snape said it loud enough for his students to hear, showing them that nobody was above his reproach and dispelling any notion that Arundel and he were in any way friendly, it was a warning to her more than anything else. A slap on the wrist for disturbing his lesson.
Arundel fished around in her pocket and gave him a handful of knutts. Snape looked down at the coins, looked back up at her with a very long face, pocketed the coins before the class could see and continued teaching. He wouldn't speak to her again, and she would set herself up in a corner of the room and ignore the rest of the class.
She set the cauldron on a fire ring and measured water to simmer before she hunted the shelves for the right ingredients. She tied her hair back cracked her neck and began. It wouldn't take long to make the draft she needed to render her students able to fly tomorrow.
She started with dried moonflower, added sloths brain and mudweed, phoenix feather and garlic and then just kept going, the potion turned red, green, caught fire and then started to turn a very light silver. She was completely absorbed with what she was doing, so much so that she didn't realise Snape had bid the class to watch her after they had cleaned up.
"Unlike your pathetic attempts, here we see a true master of her art, completely understanding the delicate balances and interactions of the ingredients. Who can tell me what she is creating?"
Griffin put her hand up, unobserved by Arundel. "Some kind of levitation potion?"
"A week one," Arundel muttered. "Can't make it too strong, need to defuse it so i'll add more mudwart to ground the base tone." She didn't even really notice who she was talking to.
"Why would mudwort act in this way?" Snape asked. "Hannam?"
"It's properties are to make items heavier or to increase their mass."
"Too much and i'll have a bunch of fat students bouncing around," Arundel muttered as she sprinkled a slither of lacewing flies in. "This will counteract it."
"Why?" one of the students asked her directly, and this broke her out of her trance and she looked up at the students. "What do you mean why? It's obvious. Are you not doing your NEWTS? Tell me why I'm doing it."
"Lacewing is used in shrinking potions." Griffin snapped at the Hufflepuff who had been made to feel very small.
"I'm going to give you five points back for that observance," Snape told her.
The potion was slowly turning blue, the steam shimmered and Arundel gave it one last stir and took it off the simmering heat. Snape turned and got her a number of small vials, and she got the class to fill them with a parapet. Half the mix was left once the sixty or so vials were full. As she worked with the sixth years Snape went to his desk and started to mark their potions, he didn't look up once.
"If we drank this how long would it take to wear off?"
"About an hour."
"How does it work?"
"You control your height by breathing. Breathing in will increase your height and breathing out will lower it. You maintain a steady height by shallow and slow breaths. Perhaps your potions master would like to let us know who's attempts today were worth winning a vial."
Snape looked up from the desk, where small tubes of potion were being tested with his wand. "I am loth to give Griffin the vile as she sabotaged her own work to get out of my classroom, but hers is the only one that would work correctly, the rest are substandard at best."
Arundel chucked a vile at Griffin, who flinched, blinked and plucked it out of the air where it spun in front of her.
"Now get out of my classroom," Snape commanded. When the sixth years were gone Snape looked her up and down, "They stayed for an hour past the end of the lesson to watch you work, perhaps there is hope for them." Arundel poured the rest of the potion into a large jar and Snape labelled it for her. "Are you going to make the antidote?" he asked.
"I'll get started on it in a moment," she told him. "Thank you for letting me stay."
He nodded and started to clean his classroom with a flick of his wand. Arundel began to work again, this time using ingredients that would neutralize the levitation potion."
"You're making this up as you go along," Snape muttered as he stood next to her looking into her cauldron. "I've never seen it done like that."
Arundel noticed how close he was standing, and how long it must have been since he washed his robes, they smelled of feet. She turned so she was facing him and ran a hand down his collar, "When was the last time you washed this?" she asked. Snape didn't pull away from her, he didn't look up at her either. She could practically see him fighting with himself. She ran his collar through her thumb and forefinger, "Do you remember the potions we used to brew? We came up with some-"
Severus smiled, his eyes flicked up to her and then his smile fell and a hollow longing filled it. "Arun," he muttered. "Don't turn me into a passing amusement because I am here and you are bored."
Arundel dropped her hand and turned back to the potion. "You were never the passing amusement, Sev, I was."
The potions master took a few steps back from her but to his credit, he didn't turn away. "I never stopped wanting you."
"I'm flattered."
"You left."
"You made it easy."
"Lilly…"
"Lilly, Lilly, Lilly Sev! Merlin's Beard, I understand how dear she was to you. I do. You don't need to explain it again."
"But you don't understand," Snape through his hands up in frustration. "You never understood."
"You are obsessed!"
"It's not an obsession," Snape shouted, like a madman possessed.
Arun threw her hands up in frustration and stormed from the potions room. Behind her, she heard Severus kick something hard and swear as he hurt himself.