Disclaimer: JK Rowling holds all rights to this story

Prologue: Sorting

Astoria went over the rules in her head.

Don't go swimming.

Don't go near the lake.

Don't go to public bathrooms.

Don't sing.

Don't play music around other people.

Don't attract unwanted attention.

Don't get too close to anyone.

Stay in the shadows.

The last rule had gone unsaid, but Astoria knew that if she did not follow all of them, she would face expulsion at best. The consequences for a part-creature being caught in Hogwarts could be disastrous; ranging from a sentence in Azkaban to a death-eater's kiss. But she would not be caught. She would not mess this up.

Astoria clearly remembered receiving her Hogwarts letter, and the relief that had followed. At a young age she had learnt that she was different. Not quite a witch. Not even fully human. She had been warned that her chances of being accepted into Hogwarts were minimal, but the letter of enrolment had finally eased her doubts.

Her nerves, on the other hand, would not be going away anytime soon.

"Don't be scared, baby sister," said Daphne, seemingly reading her mind as she leaned over to brush invisible lint off Astoria's robes. "You're ready, yeah?"

The Hogwarts Express was coming to a chugging stop. Astoria turned from the green scenery flashing steadily by to regard her older sister from the opposite bench in their train compartment.

How she wished that she had Daphne's confidence. And her normal life. And her ethereal beauty, while she was at it. Despite their identical pale skin and small, slightly slanted eyes, the two Greengrass girls looked almost nothing alike. Where Daphne had white-blonde hair and blue eyes, Astoria was a brunette with dark green, almost black irises.

"I can't help it, Daph," she admitted, fidgeting with the hem of her robe. "I keep thinking, what if people know something's off with me? What will happen if they find out?"

Daphne waved a manicured hand dismissively. "They can think whatever they want, and it won't matter. I'll be there for you, okay? And besides, the people in our house are quite private. No one will try to nose their way into any secrets, for fear of us doing the same to them."

"Daphne, you can't say 'our house'. I might not be sorted into Slytherin."

"But you will. There isn't one Greengrass in at least a hundred generations who hasn't ended up there."

Astoria grimaced. She would be the first, then. Everyone who met her was shocked when they heard her last name. 'A Greengrass, you say? But you're so…shy.' '…so sweet.' '…so soft.' She was definitely not a Slytherin by nature. Come to think of it, she couldn't really see herself in any of the houses. A terrible thought dawned on her then. What if she did not get sorted at all? Perhaps the sorting hat only worked on real wizards and witches…perhaps she would be told she had to go back home.

Astoria tried not to think about that, instead peering out the train window to watch as they pulled to a halt in the small train station. The cloudy, sporadically drizzling sky outside matched her mood.

"You're right," she finally said, standing to join Daphne in pulling down their suitcases. "I am ready." She briefly wondered if lying improved her odds of getting into the serpent house.

Daphne smirked, reading right through it. "No need to fret now, darling." She tucked Astoria's free arm inside her own and led them out the carriage. "Slytherin won't be ready for you either. Let's dazzle them all."

-:-

The hat had been cool and leathery and fell past her eyes; blindfolding and muffling her senses from the whispers of other students in the hall. It had taken a while for it to make its decision – at least five minutes, she'd been told by Daphne afterwards – and from the way it had been speaking she'd been sure that it would put her in Ravenclaw. But when it finally called out SLYTHERIN, Astoria wasn't sure whether to be relieved or disappointed.

She made a beeline for her sister at the Slytherin table, passing the rest of the evening in a blur of faces, speeches and quickly forgotten directions, while tucked up against Daphne's side.

About halfway through the start-of-year feast a Gryffindor approached them to a chorus of sneers from her fellow housemates, and Astoria cringed internally. His name was Percy Weasley, he said pompously, oblivious to the taunts around him, and he was "the new Head-boy". Astoria thanked him awkwardly when he handed her a letter and then left.

"What's that you got there, little Greengrass?" sneered a boy she didn't recognise.

His hair was whiter than Daphne's and his watchful eyes were a striking shade of mercury – handsome, she thought – yet there was a disconcerting hostility in his voice.

"None of your business, Malfoy," snapped her sister on her behalf. "Sod off."

A girl, also second year, with dark pigtails and a pug-like face, looked up at the remark. "What's got your wand in a knot, bloodwhore? Mummy been flirting with strangers again?"

Malfoy snickered with the two brutish cronies beside him.

Daphne ignored them all and blocked their view with her shoulder as Astoria, more nervous than she'd been before, resumed opening the letter.

Miss Greengrass, it read in recently dried ink.

Congratulations on your sorting!

In light of your condition, special measures are being conducted to ensure that your stay at Hogwarts is as hospitable as possible, including a private bathroom to be made accessible to you from behind your bed. However, we must still implore that you follow these three guidelines, for the sake of not only your own but the students around you:

Take care when approaching water bodies

Do not sing or participate in musical activities.

Report regularly to the hospital wing for your weekly treatment from Professor Snape.

Wishing you a prosperous year,
Professor Albus Percival Wilfrick Brian Dumbledore

Astoria turned to Daphne, who had been reading over her shoulder, and the two shared a look.

It was only when they were getting ready to leave that Astoria glanced up. She realised that it was not only the letter that was giving her a prickly sense of foreboding, but the pair of steely grey eyes that had been watching her the whole time.

-:-

Later, when the first-years had been escorted down to the dungeons by the prefects, Astoria had surveyed the room that she was going to be living in for the next seven years. Her bed lay furthest from the door, and she knew that behind the four-poster's curtains she would find a secret tunnel, leading to a bathroom just for her.

Green light shifted and rippled eerily across the walls; a reflection of the lake that a ceiling-to-floor window looked in to. The view beyond was even stranger. Columns of seaweed swayed slowly through the water, glinting like broken glass in the fading twilight. The silver-tipped tail of what must have been an eel slipped out of sight.

Astoria went up to the window and pressed her fingers against the glass, feeling a pull from deep within her gut as she did so. She imagined to herself that just beyond the underwater forest, a village of merpeople lay waiting.

Her brethren.

What would people say if they knew they had a siren in their midst?

She caught sight of her reflection in the glass. Dark eyes stared back at her, holding a secret as old as myth and magic itself. No one can know, they said. Stay in the shadows.

Then the final light of dusk faded, taking her mirrored-image with it. Astoria turned away from the depths of the waters she longed for, and allowed herself to be engulfed by Slytherin's darkness.

Hi everyone! This is my first Harry Potter fanfic, and I really hope you guys like it because I have high hopes and lots of ideas. Please favourite or follow if you like, and feedback is greatly appreciated – even critics. So yeah, I hope you enjoyed this chapter on Astoria's first year, we'll be taking a bit of a leap in time from here on :)