Work Text:
“Kacchan, for the last time, I don’t want to be rescued.”
Katsuki’s eye twitched. His deep black tail nearly blended into the darkness of the sea cave. If it weren’t for the slow transition into peachy pale skin just above his waist and the neon orange outline around his anxiously fluttering fins, he would have excellent camouflage. His flashy golden spear and bright hair would have caught attention, as well. His black gills stretched into long neon-orange-tipped ears, and his coral-red eyes burned with frustration.
Izuku knew that even if Katsuki had intended to be covert in his rescue, he certainly wouldn’t have been successful. He was too brilliant, seeming to absorb light and bring it with him wherever he went, even in the kingdom’s darkest dungeons.
Besides, there were dozens of guards posted outside of this cave, and thousands of civilians who were intent on Izuku’s death.
“What have I done any of this for?” Katsuki growled—no, pleaded. “What have I protected you for all of this time? You’re my prince . Marithor needs you.”
“I am secondborn. I am not the heir. Merely a spare,” Izuku said, then smiled. A little sad, but resigned. “Kacchan, you’ve completed your purpose. You saved me for this .”
“I don’t accept that! That’s fucking bullshit, and you know it!” Water bubbled around Katsuki’s hands, white hot with magical magma. “Izuku, we trained our magic for years. Years. We read those damn crusty old spellbooks. We don’t have to submit to this. We can overpower any of these idiots.”
The prince uncurled his tail and swam up to the bars of the prison cell, calm before Katsuki’s enraged expression and flaring nostrils. He had known Katsuki since they were still sleeping in the clamshell beds of the hatchery.
He felt like it had been longer since they had been clustered eggs, but he knew that wouldn’t have been true. Only after their hatching had Katsuki been assigned his destiny by the King and Queen of Marithor—to be their secondborn son’s guard.
“Leviathan desired a sacrifice,” Izuku said. “He asked for me by name. I am happy to give my life for my people.”
“No, you’re not,” Katsuki said. “That self-sacrificing prince bullshit rhetoric is not going to work on me. I know you, ‘Zuku. I know that you’re not happy. You’re miserable . This is unfair! You want—you want to live . You want to escape this place with me! I know it!”
Izuku’s dark green tail faded to a white, elegant betta fin. The blue glow of the bioluminescent plants crawling up the walls of his cell made his iridescent scales flash silver and sky blue. His eyes flashed similarly at Katsuki’s protests.
“It does not matter what I want to do, or how I feel. I am doing the right thing . If I don’t, Leviathan will devour our city.”
Katsuki’s expression crumpled. It cut Izuku deeply.
“You’re an idiot. This isn’t the right thing. This is—this is the easy thing ,” Katsuki said, then cut off Izuku before he could protest. “ For everyone else . Your parents are doing this because it’s easier than standing up to Leviathan.”
“Scores of our people perished when they tried to ‘stand up’ to him.”
“They have also done so while compliant,” Katsuki said. “There’s no point to this. It’s an endless fucking cycle—”
“You’ll be free!” Izuku shouted, finally breaking his princely calm. Tears shone in his eyes and salted the water around his floating curls. “Kacchan, you’ll be free. You’ll finally be able to choose your destiny. You’ve always wanted to explore the rest of the ocean, to see the tropical coral reefs, to see the famous trenches, or to explore freshwater streams! Or find a jellyfish bloom, or—or—
“ You wanted that, Izuku! You wanted those things! You read about the coral reefs and jellyfish blooms and wanted to see them with me !”
“You won’t be tied to me anymore!”
Katsuki’s mouth was pressed tightly. It was time for him to be the composed one, his shoulders speckled with white and black scales rolled back.
“There it is.”
Izuku sobbed, pressing his hand to his mouth. “Kacchan, I can’t—I can’t live knowing that I don’t—I don’t have a purpose, and yet, you must be chained to me. I love you too much. You’re more than my bodyguard. You’re my...”
My…
Katsuki reached through the bars and grabbed Izuku’s hand. He pulled him closer so that their faces were inches apart.
“Izuku, I would have chosen you regardless. You are my destiny, not because of what the king and queen did, but because I always would have wanted you.”
Izuku stared at him. His eyes were soft and full of wonder, clutching Katsuki’s hands and pressing them to his freckled cheek. And then bitter, crushing despair fled the light out of them. He gritted his teeth and sobbed, chest heaving.
“It’s too late. Leviathan—”
“Izuku, we can—”
“I can’t leave. Leviathan will kill you, too. He’ll destroy Marithor and everyone in it.”
Katsuki took a deep breath, his black gills fluttering. After a moment, he spoke.
“If it were me on the chopping block…if he had asked for my name instead, what would you have done?”
Izuku recoiled, but Katsuki held him fast. “ Think , Izuku. Use that brilliant nerd brain of yours. We have one last night. What would you have done if it were me? ”
. . . . .
Izuku was dressed prettily for his end. A silver gorget necklace with intricate designs and inlaid pearls, braces around his biceps, and mother-of-pearl handcuffs behind his back. His tail was bound together in black netting so that he couldn’t unfurl it to swim, his long, iridescent betta fins rippling in the current, splayed out behind him.
His parents had watched with stricken, pale faces as the guards and servants drew him out from his cell. Izuku had surrendered to the jewelry, the pampering, the handcuffs, and the netting.
His older sister, the heir and crown princess, took his face in her hands before letting the guards lead him out to the cliff.
“Our parents are cowards,” she hissed under her breath. Her black hair was long and wild around them, a small cloak of protection from the stretching, dark depths of the ocean.
“Hush. You will be queen soon,” Izuku said. “You can save our people from Leviathan.”
“Just say the word,” she pleaded. “Just say the word, and we’ll be done with this. I will help you escape. I want to save my people, believe me, but I don’t want to do this. I don’t want you to die.”
Izuku knew his sister meant it. Momo didn’t say things without meaning them. But at the same time, he knew she couldn’t pick between being a good sister and being a good princess right now. This was the only thing she could think to absolve her guilt—to have Izuku choose for her.
And in a way, he was grateful for that. Grateful to be presented with even a shadow of a choice—a mockery of a choice.
He just shook his head. He didn’t want her to feel guilty for this. It wasn’t her fault.
“It’s okay. I just want our people to live.”
She burst into tears. “I love you.”
“I love you, too.”
“Do me a favor?” Izuku swallowed.
“Anything.”
“When I—” He pressed his lips together. “When I’m gone, let me be the last one. Help our people leave this city. Migrate far to the south, where the waters are too warm and shallow for Him. I—I have notes in my study. I read books from our ancestors—timeworn journals that He has banned from our cities. One of them mentioned such a possibility.”
She paled. “Izuku. That may be why He chose you.”
“I know. Do better than me at keeping it discreet from His all-seeing eye.”
“Daughter, it is time,” their father ordered. Distant, pale.
Izuku had seen this face on him before. When he had ordered an execution for someone. He had never had the stomach for it.
“Goodbye.” She pressed a kiss to his forehead. Soft, warm.
And then she straightened, swimming back to her parents, her soft, petal pink betta tail rippling in the currents.
He didn’t look at his mother and father for very long. He understood their despair, understood that they did love him, that they didn’t want this to happen, but he felt the reluctant stirrings of resentment clash with his abiding loyalty. He did not want to see them in this state any longer. He wanted to remember their strong moments for the rest of his life. Not this.
He peered over the cliff, a small, tied-up merman against an unending blue and black sea.
When Leviathan rose, the word ‘coward’ was banished from his mind.
All other thoughts dissolved like sand clutched in webbed fingers in the tides.
It was only Him left.
He was immense. Vast. Infinite.
Izuku was like a speck of algae facing the towering baleen teeth of a whale. A great dark form rose from the depths and blotted out the sun’s rays filtering through the water. The sea swirled, infinite tides and currents interrupted by a deep red body covered in barnacles and seaweed.
Izuku’s expression was calm, his gaze set and focused. His heart pounded.
Something deeply biological inside of him fought to wriggle back, to break his bonds, to swim for his life . This was an ancient thing, an old predator. A cruel beast that his ancestors had made far too many blood-soaked deals with.
“Leviathan,” he said out loud. “I have come in exchange for you sparing Marithor. As you requested.”
Continents crashed together in his mind, deep and echoey, and Izuku realized belatedly that this was a chuckle. His body shook.
“A morsel such as you would not satiate my hunger.”
“Please!” Izuku pleaded. “Please, Great Leviathan. Please spare the city a little longer. We will continue to submit to your demands as you see fit. We will continually bring you all of the sea beasts you desire and the mer that you name. Just—please. Please . Just take me.”
“As you wish, little creature. Wrapped so nicely for me. Such a brave, trembling thing. Do you not realize how infinitesimal you are to me? You, so little, could not begin to wonder at the eons I have seen. I witnessed worlds born and slaughtered, stars bursting into new cosmos. I enjoyed the killing of them until I tired of it. You pray for the embarrassingly short and futile lives of creatures who would be lucky to make a mark on the world as ephemeral as writing in the sand.”
Izuku straightened. He stared into the burning singular eye of Leviathan, a round, black, beady thing a hundred times Izuku’s size, filled to the brim with the very cosmos He spoke of.
The merfolk loved the stars and moon almost as much as they loved the sea, fleeing to the surface to make patterns and stories out of them, chart them, and discover more about them.
“There are trillions of stars in the sky, Great Leviathan. So many that we cannot possibly chart them all, and many we cannot even see. And yet, they are all brilliant. That is how I see my people. Marithor is a galaxy that may one day die, but now is not that time. For them, I give my life.”
Silence for many moments. Izuku worried that he protested too much—he doomed his people. He should have nodded his head and agreed that their lives were small and meaningless compared to His.
“Your supplication spares them another day, young prince.”
His great maw opened. The ocean churned and cracked apart, hundreds of whirlpools swirling where His lips separated. Every mer on site gripped each other, their spears, and the nearest boulder or coral to stay put where they were. Izuku began sliding towards Leviathan’s baleen teeth.
Dozens of royal guards began dumping fishnets loaded with the carcasses of fish, sharks, whales, and manatees over the cliff down His hungry gullet.
He heard more continents crashing and quaking in his mind—cackling. Cackling .
And then Izuku was sucked in.
He tumbled, end over end, into the darkness between the sharp teeth. His sister screamed.
The shred of light snapped together.
An infinite space. A galaxy reeking of dead fish.
Izuku’s chest constricted. His mind shrank into a tiny point, his breaths panging against his own ears—too fast, somehow. Everything was too fast, his gills filtering, his heart hammering, his throat closing—
This was it. This was it. The terrible, dark space where his nightmares always ended. Where every mer’s nightmares ended.
And then the sun exploded.
And Kacchan, of course, was in the center of it. He held up his fist, smiling like a maniac, and released a blast of magma.
In Kacchan’s light, reality struck Izuku all over again. Yes, Leviathan was old and massive, but He was still a creature. One who hungered, who demanded to be satiated, with flesh and bone. Izuku could see the fleshy pink of its mouth, the veins the size of underwater caves webbing the palate.
He could bleed.
He could burn.
“Kacchan!”
“Hey,” Katsuki grinned. “Ready to wreck this thing?”
Izuku smiled back, his anxiety lightening. Oh, this boy. This boy was sunshine incarnate.
“I don’t buy it,” he had told Katsuki in their whispered, desperate plan. “ I don’t buy that He is all-knowing, all-powerful. He may be ancient, but he was a beast born in the waves like the rest of us. He is no more than a very old liar.”
Izuku had convinced his parents that morning to send in a load of carcasses with him. He had claimed it would assist in satiating Leviathan, but in reality, the carcasses had disguised Katsuki sneaking inside of the maw with him.
Additionally, each large body—the manatees, the whales, the seals—they had a lot of blubber.
“Aim for the food, Kacchan,” Izuku shouted.
“Don’t have to tell me twice.” He released blasts of boiling heat, decimating the blubbery creatures. Izuku grimaced, feeling for the creatures, but their bodies had long been dead and gathered for this very purpose. Blasts of heat lit up the inside of the mouth, and hot, boiling oil splattered against the walls and roof of the mouth, burning bright red and black patches.
Lightning crackled around Izuku, thrumming through his silver braces and frying his net to shreds. He stretched his tail, relieved.
The magic of the merfolk had always been powerful, but his people had forgotten how to use it to defend themselves. Under Leviathan’s influence, they had relegated it to small, practical, everyday uses to avoid his ire.
Izuku and Katsuki had been reading the forbidden texts for years, honing their offensive skills carefully.
That was the true reason Leviathan had wanted him dead, he knew.
What would be the point, fighting an ancient, impossible being?
The point was to make it hurt. If a mer avoided sea urchin despite being perfectly capable of crushing it simply because of its spines, then Izuku would become Marithor’s spine.
“WHAT ARE YOU DOING?”
“Fucking you up,” Izuku said simply. Kacchan laughed wildly, blasting more carcasses and watching their boiling blubber oil splatter.
“You motherfucker! You think I’m just gonna let you eat Izuku?” Katsuki roared. “If we’re gonna go down, then we’re gonna go down with you screaming!”
A great roar shook the world and tore it apart.
“We promise to stop doing this if you never, ever threaten us or even speak to us again,” Izuku shouted, and shot a bolt of electricity at the roof of his mouth.
“INSOLENT BEASTS! I HAVE WATCHED SOLAR SYSTEMS CONCEIVE AND DIE!”
Katsuki howled with laughter as he pressed white hot hands into his soft palate. “Please, you can barely get off your big ass to hunt your own damn food!”
Oil bubbled and separated from the sea’s salt water, filling its space rapidly. His skin burned and blackened, magma releasing and cooling to rock.
The beast opened his mouth. Cool blue filtered into the black.
“I WILL DEVOUR YOUR CITY!”
Izuku looked out at the open blue with longing, then swam further into his mouth.
He swam deeper, Katsuki following him and shooting off blasts of heat. He pressed a hand against the wall of the creature's throat and sent a blast of lightning into its flesh, burn marks smoking around them.
“I’ve been studying sketches of you for years. I imagine your anatomy isn’t too different from a whale’s. My friend and I are going to blast a hole through your brain casing and find you,” Izuku said. “We’ll find your brain. And then I’m going to electrify it. You’ll lose pieces of yourself before you can blink.”
In truth, Izuku knew that would take energy and time that they didn’t have. Katsuki’s gills and chest were heaving, and a coppery taste filled Izuku’s mouth. By the time they even found the creature’s brain, they would certainly have starved or fainted.
The creature was moaning in pain.
Izuku almost felt bad.
Almost.
“Fine. I tire of this. Leave my body at once.”
“If you try to eat Marithor, then you eat us ,” Izuku said. “And we will finish what we started.”
. . . . .
Leviathan was gone.
Izuku was limp on the rocky cliff’s ground, Katsuki in his arms.
They were alive. And Leviathan was gone, leaving a blue open space stretching for miles.
Izuku’s family and the royal guards stared at them with open shock and confusion.
Izuku and Katsuki met each other’s eyes.
Katsuki smiled crookedly.
“That was the craziest shit we’ve ever done. You brave, stupid, shitty nerd prince.”
“Not as crazy as you.” Izuku beamed.
Izuku cupped Katsuki’s face and kissed him.
It was like kissing the sun. Bold, hot, and brilliant.