Chapter Text
Immediately, Regulus could feel it, and he knew that James could too. As soon as Sirius introduced the two of them, something clicked. James was like him. He was a merman.
Ten years ago
Regulus and Sirius were on a trip together, Sirius being six and Regulus being five. They were in Australia for a business meeting with their parents. It was in Queensland, right on the gold coast. Their mother often sent them off to play in the sand together with their older cousin who was local to there. Being with her was fun, sure, but she could be irresponsible, to say the least.
“Andromeda!” His mother would shout, “Me and Orion will be back in approximately four hours, keep them here until then.” She commanded, tone firm and voice sharp.
Regulus splashed in the shallow area of the waves, hitting his brother with a small wave of water. The two boys giggled as their mother faded into the distance, and Andromeda approached them with a skip and a cheerful grin, as always.
“Boys, how would you like to go out there?” Their cousin, who was only thirteen at the time, asked them, crouching to get on the boys’ level.
She was pointing towards a large island in the distance, with bright greenery and a large volcano which towered over the land. Clouds seemed to be circling the island, touching nowhere else in the bright sky except for there specifically.
“Yes! Do you have a boat?” Sirius asked with joy, pulling himself and Regulus to stand.
“Why of course mister Siri!” She ruffled his wet, black hair and took both his and his brother's hands, walking towards the docks.
“Mother said we should stay here,” Regulus said in his small, timid voice, gripping onto his cousin's larger hand.
“Well she won’t find out then!” Andromeda smirked and placed a finger to her small lips, winking at the boys as they approached a large boat, and lifted them both one by one onto it. She showed Sirius how the ignition worked, whilst Regulus cautiously laid back, staring down into the shark-infested waters.
It took them roughly thirty minutes to reach the Island, and by that time the sun was already beginning to set. Andromeda reached for her phone in silence as Sirius went on a tangent about how much Australia was superior to England, and that he was going to move there in the future. Andromeda hung up the phone and turned towards the boys, crouching with a warm smile which filled Regulus with bliss.
“Your parents said that we can all camp here tonight, isn’t that fun?” She said, softly. Sirius began to quite literally jump with joy, whilst Regulus swallowed harshly.
“I-Is it safe?” He asked, shakily.
Andromeda ruffled his hair, reassuringly. The three of them then traversed through the green forest until they were at a space which seemed safe to spend the night. It had no rocks for almost a mile, as that was where a nearby spring was.
Sirius fell asleep almost immediately, and Andromeda followed soon after, but no matter how many times little Regulus tossed and turned, he couldn’t shake the ominous feeling which surrounded that place. He pulled his petite body to stand, and quietly began to walk away.
He always felt that his head became clearer when he went on a walk, sometimes he would sneak out of his bedroom window if his parents were arguing, him and Sirius would sit on the roof and speak about the future.
He walked for around twenty minutes north before he reached a small spring. A small, cold river ran through the rocks, quiet and clear. Near the edge, there was a little opening in the ground, which seemed to be an entrance to a cave below. The rocks around it were slippery, and if he were not careful, he could fall straight in.
Regulus swallowed as he stepped on one of the rocks, his small legs stumbling a bit before the water which ran on the rocks caused him to fall down. He began to panic, he couldn’t swim, and his small legs meant that he couldn’t stand if it was deep. Luckily, he fell down a dry passage, which was covered only in sand.
“Ow,” Regulus sighed as he sat in defeat, “Why did I do that?” He quietly mumbled to himself as he wiped sand from his trousers.
There was no way that if he screamed anybody could hear him from that far, so Regulus thought it would be better to wait until sunrise so that he could see and his family would be awake. Regulus wandered aimlessly around the cave, gasping at the sparkling, black rocks which covered the walls.
Suddenly, Regulus heard a loud yell from behind him, which he recognised well. That scratchy voice was Sirius’, he knew it. It was followed by a female screech, who could only have been Andromeda.
“You’re here!” Andromeda sighed with relief as she pulled Regulus’ small body into a tight hug. When she released him, she checked that he was okay before brushing her and Sirius off.
“Why are you here?” Regulus asked, still a bit confused from the whole situation.
“Sirius woke up after hearing that you left, and we assumed that you would head for the water, you do love climbing after all.” Andromeda ruffled his hair softly, before taking both of the boys’ hands and walking further into the cave.
“I can climb back up and get a rope!” Sirius declared with pride, using his hand which wasn’t attached to Andromeda to bang his own chest, brimming with confidence.
“What rope?” Andromeda giggled.
“You can’t climb sand, idiot.” Regulus tightened his grip on Andromeda as they walked for about ten seconds into a clearing.
They were inside the volcano, it was empty other than a small pool of water which glistened in the small peak of moonlight above. The full moon was peaking slightly outside at the top, it was halfway showing. Andromeda crouched down, examining the water.
“We can swim out underwater, we should give it a try now before the tide comes in and the fishies wake up,” She said with bravery.
“Yes! Let’s go, Reggie!” Sirius said with a wide smile, probably beaming with adrenaline.
“I-I can’t swim!” Regulus shouted, the room spinning as he stared at the deep water in front of him.
“It’s okay Reg, I can carry the two of you, you know I’m a part time babysitter, I have to carry kids to bed all the time, so I’m strong.” Andromeda comforted Regulus as tears welled up in his wide, grey eyes.
He gave a small nod as Andromeda slowly climbed into the water, her feet not even close to touching the bottom. Sirius was about to bellyflop into the water, before Andromeda scolded him and carried him herself. She had a tight grip on Sirius with one arm and held her other out to Regulus.
Regulus swallowed, staring down at her pale hand. He sat down at the edge of the water and looked up, the moon was a few seconds away from being full. He closed his eyes and took a deep breath before dropping into the water. Within seconds, Andromeda grabbed him, and was holding onto the both of them.
Andromeda held five-year-old Regulus and six-year-old Sirius close as they floated together in the warm, quiet pool tucked inside the volcano’s core. Above them, the full moon crowned the open crater, its light spilling down and setting the water aglow with drifting sparkles that rose gently around them, as if the air itself had started to breathe.
The three gasped at the seemingly magical sparkles which surrounded them, before Andromeda sharply told them to close their eyes and take a deep breath, which the two boys did quickly.
Andromeda dived down, holding the boys tightly before they finally could breath again. They inhaled the air greedily when they reached the top, and began to swim quickly to the shore again. It was obvious that it was hard for them to sleep that night, Regulus felt himself drifting off again, and he allowed himself to drift to sleep, tired and confused, with a tingling feeling in his heart.
The next day, after returning back to the island, Regulus and Sirius’ mother told them to pack quickly as they were leaving the next day. The boys obediently listened, rushing to Andromeda’s house to collect their things. Andromeda was sitting on the sofa, staring at seemingly nothing.
“Dromeda?” Sirius asked, rushing towards her.
“Boys, can you come with me to the pool?” Regulus and Sirius nodded, confused. They went to the pool which was in her back garden,and Andromeda stepped into the shallow end.
The boys stared, breath caught in their throats, as the pool began to shimmer like starlight had fallen into it. Andromeda stood at the center, her eyes wide with something between wonder and dread. The surface of the water rippled unnaturally around her, glowing brighter by the second, until it surged upward in slow motion, almost like it had chosen her. The strange water wrapped around her legs, then her waist, then her chest-sparkling and sludgy, thick like liquid moonlight.
She didn’t move.
Regulus let out a quiet, strangled sound. Sirius clutched his hand, but neither of them could look away.
Then, in one blink, the water swallowed her whole. A heartbeat later, it vanished, like it had never been there at all. Andromeda dropped into the pool with a splash, her head breaking the surface with a gasp. But she wasn’t the same.
Where her clothes had been was now a gleaming, orange mermaid tail that glistened like fire under the full moon. Her top had turned to a delicate shell bra that clung to her like it belonged there. She stared at herself, horrified, lifting her hand out of the water to see it still sparkling faintly.
Regulus fell to the stone floor with a thud. His face went pale, and his small hands trembled violently in his lap. “What… what just happened?” he whispered, though no one answered.
Sirius hesitated only for a second before stumbling toward the edge of the pool.
“Sirius, wait!” Regulus cried, reaching out. But Sirius didn’t stop.
He jumped in feet-first, water splashing high into the air. He kicked clumsily at first, paddling like a dog just to stay afloat. But as the water hit him, the same strange shimmer started to rise around his body. It crawled up his legs, up his back, until the glow consumed him, and then, like before, it vanished.
Where his legs had been, a sleek, powerful blue tail now flowed behind him, catching the light in every flick. His upper half remained bare, his hair slicked back and dripping. He looked at his reflection, blinking, speechless.
“Regulus,” Andromeda called gently, her voice tight, urgent. She swam closer to the edge, her eyes locking onto his. “Come here too.”
“No,” he said, his voice cracking. “I don’t want to, I don’t want that.”
“I know. I know,” she said, a tremor in her voice. “But we can’t leave you behind without checking.”
Regulus stood frozen, staring at the glowing water like it might bite him. He crept forward slowly, dipping one foot into the pool, and in that instant, Sirius shot forward and yanked on his ankle.
Regulus screamed as he tumbled in, his arms flailing. The second he hit the water, it was as if the world changed. The warmth swallowed him whole. His vision blurred with sparkles, and that strange sludge-like magic began to curl around his body.
He gasped and pulled himself to the surface, panting hard. But when he looked down… he wasn’t the same anymore.
A seasick-green tail shimmered beneath him, long and glistening, curling in slow waves. His legs were gone. His clothes were gone too. Just like Sirius. Just like Andromeda.
“I’m so sorry,” Andromeda whispered, pulling them both toward her, her arms wrapped tightly around them. She held them as if she could protect them from whatever had just happened. Her voice broke. “I didn’t know. I didn’t plan this…”
She buried her face into their wet hair, sobbing quietly.
“It goes away when we dry, okay? We look normal again. Just avoid water. As much as you can. Hide it from everyone. And when you go back to England, I’ll figure this out. I swear I will.”
Regulus clung to her. Sirius stared blankly at the sunlight above, his face unreadable.
Andromeda pulled back just enough to look at them both. Her voice was barely above a whisper. “Come back to me as soon as you can.”
She kissed their foreheads, ruffled their hair;but the comfort didn’t land. They were too stunned, too changed to say anything.
“I love you both,” she whispered.
And that was the last time Regulus saw her, for ten long years.
The years passed like a dream they never woke from. Back in England, life resumed around them. School, family, their parents’ expectations. But none of it felt the same.
Not after that night.
They learned how to avoid water like it was poison. Showers were brief. Rainstorms meant staying inside. Pools and lakes were out of the question. And if something happened, if the change came, they knew to hide, to wait until they dried off, to pretend like nothing had happened.
No one ever found out. Not even their parents.
By the time Regulus was fifteen, they had begun to understand that it wasn’t just a curse, it was power.
Regulus could control water. He could move it with his mind, bend it like it was alive. He could twist it into shapes, make it rise from a puddle and hover in midair.
Sirius, wild and untamable as ever, could evaporate it in an instant. He learned to summon heat from his skin, to boil a glass of water just by touching it, to turn a foggy morning into a clear one with nothing but willpower.
Andromeda, still in Australia, still far away, could freeze it into crystal sculptures with a flick of her hand. She told them this in hushed phone calls, always careful, always watching her words.
Sirius had finally had enough.
Regulus could see it in his brother’s face the moment he stepped into the room, his shoulders drawn tight, lips pressed into a trembling line, eyes glinting with something raw and final in the soft glow of the moonlight. He didn’t yell. Didn’t curse. Just stood there, shaking, with something fragile barely holding him together.
“They’re going to kill us, Reg,” Sirius said, his voice flat and quiet, like he’d run out of ways to scream it. “We can’t stay here anymore.”
Regulus didn’t answer. He didn’t have to. He just stared at the bruise blooming beneath the sleeve of his jumper, the echo of their father’s grip still stinging against his wrist.
They both knew it had been coming. It wasn’t just the shouting, or the punishment for things they never did, it was the emptiness in their parents’ eyes when they looked at them. Like they weren’t sons. Like they weren’t even real.
Sirius crossed the room and dropped a wad of cash onto Regulus’s bed, crumpled notes he must have taken from their father’s drawer. “We’ve got maybe an hour before they’re back. Pack what you need. We’re leaving. I mean it.”
Regulus blinked up at him, stomach tight, heart hammering. “Where?” he asked, voice barely a whisper.
Sirius didn’t hesitate. “Australia. We’ll find her.”
And Regulus knew exactly who he meant.
Andromeda.
She had been thirteen when it happened. Regulus was five, Sirius six. The three of them had been in that strange cave pool, the one under the volcano on that business trip their parents insisted on. He remembered how the moonlight had poured down through the open crater, turning the water into something which made no sense.
That night bound the three of them together forever. Mermen. A mermaid. And no one else could ever know.
Ten years had passed since. Andromeda had disappeared not long after, telling the family she was going to university in Australia. But Regulus remembered the phone call she made, voice quiet through the crackling line: “ When you’re ready, I’ll be here. There’s water here that understands us. There’s space to be who we are .”
He hadn’t thought that moment would come so soon.
He packed without thinking. Clothes. Toothbrush. The journal he and Sirius kept, the one where they logged every change, every time the sea called to them louder than it should’ve, every time moonlight made their skin itch with magic. He hesitated over the shell necklace Andromeda had given him years ago, then tucked it into the side pocket of his bag.
The house was silent when they left. Not peacefully silent, but hollow. Like it had already forgotten them. Regulus shut the door behind them as gently as he could. He didn’t look back.
Outside, the street was drenched in silver. The full moon hung low and heavy above them, spilling light across their shoulders. Regulus felt the familiar pull at the back of his knees, like something inside him was waking up. His legs ached faintly.
Not now, he told himself. He could control the pull of the moon after all this time. And that wasn’t going to change now.
Sirius called a taxi from the payphone at the corner, too paranoid to use his mobile. When the car arrived, they slid into the back seat without speaking. The driver didn’t ask questions. He didn’t need to.
At the airport, Sirius handled everything. He paid in cash, voice low, face unreadable. Regulus followed his lead, grateful for the quiet, for the way his brother moved with a strange kind of steadiness now that they were in motion. He always did better when he had a direction.
The flight felt endless. Regulus couldn’t sleep. He sat by the window, watching clouds drift by, thinking of the ocean far below them. He could feel it pulling at him, even from this height. whispering to him through his blood.
Every now and then, his legs twitched, and he had to clench his jaw to keep from panicking. He couldn’t shift here. Not in the air. Not in front of strangers.
Sirius slept fitfully beside him,leaning on Regulus’ shoulder, muttering things under his breath. Regulus didn’t wake him.
When they landed, the heat hit him first, thick and golden, rich with the scent of salt and something green. The air felt heavier here, but not in a bad way. It felt... alive.
They moved slowly through the terminal, exhausted, quiet. Regulus scanned the crowd for her, not sure if she would even be here. Not sure what he’d do if she wasn’t.
But then-there she was.
Andromeda.
Standing by the rail, hair twisted back in a loose braid, wearing a white shirt that billowed in the breeze from the open doors. She looked different. Older. Stronger. But her eyes, those eyes, were exactly the same.
She saw them. She Froze. And then she ran.
Regulus dropped his bag just in time to catch her as she wrapped both him and Sirius into a bone-crushing hug. She was crying, and laughing, and saying their names over and over.
“My boys,” she whispered, her voice cracking. “You made it. You’re here. You’re safe.”
She took them home, to her home, tucked into a cliffside where the sea roared beneath and the windows faced the horizon. It was warm, and quiet, and smelled like salt and rosemary. Regulus felt his shoulders relax for the first time in what felt like years.
There was a door behind the laundry room, hidden by a cabinet. Behind it, a narrow staircase carved into the stone led to a cave, half open to the sea, half sheltered by rock. The tide came in and out through a tunnel only visible during low tide.
“This is where I swim, it’s connected to the moon pool. Our moon pool.” Andromeda said softly. “You can use it too. It’s safe here.”
Only the three of them knew. No one else ever would.
That night, she made dinner, real food, warm and simple. Afterwards, she sat with them in the living room, listening without interrupting as Sirius explained everything. Regulus barely spoke. He just watched them both, heart full, throat tight.
Later, in the spare room she gave them, Regulus lay in bed with Sirius’s steady breathing beside him. The windows were cracked open, and the sound of waves filled the space between them. Moonlight poured across the floor.
His legs ached. The sea called.
But he was safe.
Sirius stirred and whispered into the dark, “We made it.”
Regulus didn’t reply.
He just reached across the bed and held his brother’s hand, tightly, firmly, like a promise.
Like an anchor.
Like the tide would never take them from each other again.