Honestly, this was supposed to be a Twitter thread but now we're here lol. If you notice that at least at the very beginning a lot of the sentences are shorter and the shit moves more quickly it's because this was only supposed to be a couple hundred words. Tomorrow I'm going to try my hand at it instead I think. Hope you enjoy!

Eren knew he was in trouble as he saw the red sky that morning. The crews' pleas fell on deaf ears as the captain commanded they set sail to the next port. Eren did as he was told, but he looked towards the sky often, worry ever-growing when the clouds darkened above them by afternoon.

It wasn't long before the storm struck them. Gigantic waves towered over their ship, pushing and pulling at them like a toddler fighting over a toy. They scrambled to hoist the sails, secure the gunports, and bucket out the water quickly filling the keel. It was no use.

Waves slammed into the ship, sending half the crew overboard. The wind and rain tore at their clothes, Eren struggling to hold onto the sails as they attempted to raise the wet fabric.

"Get your backs into it! Heave!" One of the officers commanded, joining the remaining crew in pulling at the ropes. Eren grunted, hands burning at the coarse feeling of the rope.

Another wave crashed into the deck, pushing Eren into the side of the ship. Pain erupted in his skull, his head colliding harshly with the clipper. Green eyes looked over at the bow. He gasped.

Large jagged rocks jutted out of the dark water, an ominous tower looming over the ship as they sailed ever closer. We're gonna crash! Looking over at the helm, his face paled when he saw that the helmsman was on the floor, seemingly knocked unconscious by a wayward beam. Even if Eren sprinted, there was no way he'd make it to the helm in time to steer them away. If he could, the waves would most likely knock him clean off the boat.

We're crashing, there is no way to stop it. What do I do? Think! Think! Think! Eren looked around wildly, trying his best to ignore the screams of people he considered friends as they were roughed up by the waves and flung overboard. I can jump overboard, but the waves will just push me into the ship or the rocks. If I just wait for the ship to crash, I might be able to climb onto the rocks or into he more shallow areas and wait out the storm. Fuck, why didn't the captain listen! He knew why. He was a rich kid, the son of a merchant and just bought his very first boat. He had clearly never lived at sea, and didn't respect the power she had. And now we're all gonna die. Before Eren could come to a decision, he realized that a wave had pushed at the stern. The ship surged forward, smashing into the side of the tallest spire. The ship screeched as the wood buckled and snapped, rising out of the water at the force of the sea pushing against it. Eren was flying before he could realize what had happened, grabbing at anything as he fell into the murky water below.

It was cold and dark. Eren couldn't tell which way was up or down as he desperately clawed at what he hoped was the surface. His lungs squeezed painfully in his chest, skin prickling in the cold water. He had nowhere to go, his limbs refused to move.

I'm going to die.

He couldn't hold it back anymore. He sucked in for air, choking when all that came through was salty water. His lungs were filling up and his vision was spotty.

Help, please…

His mind was shutting down as he slowly sunk deeper into the abyss. As his eyes closed forever, he could swear he felt hands on his face.

An angel taking me to heaven…

That was the moment Eren Jaeger died.

So naturally the moment he woke up was rather confusing.

He groaned, his head throbbing, pelvis and legs aching. Am I… dead? His last memory was drowning, swallowing gallons of water and feeling the cold of the sea embrace him.

He opened his eyes.

It was dark, he could barely see some basic shapes that seemed to indicate he was at least inside something. He tried to sit up but the strangest feeling washed over him.

He was floating.

He was underwater.

He was breathing.

What the fuck?

Eren spun around, kicking out to try and swim upward, hoping to find air. However what he wasn't expecting was just how fast he went, slamming into the rock above him, hurting his already tender scalp a second time.

"Fuck!" He exclaimed, quickly covering his mouth when he once again realized he was underwater. The memory of the painful way his lungs contracted when he swallowed mouthfuls of water constricted his chest. He was panicking. What do I do?! Fuck not again where am I? Where's the surface?! Emerald eyes darted around, searching for anything but he couldn't see for shit. During this time he failed to recognize that his legs were not moving separately but as one.

He smacked the stone with his fist, hoping beyond hope that something would give way. He needed air, he needed to breathe. The brunet cried out, air bubbles floating passed him as he blearily watched.

Is this hell? Am I to die, drown, over and over?

Eren gasped, body demanding he breathe and he waited for the suffocating feeling to start as water was pumped into his lungs.

That feeling never came.

One breath. Two breathes. He breathed in a third time, slow and steady.

Well, that's not right.

"You're awake." A voice suddenly spoke. Eren whirled around, looking for the source and feeling his stomach flop when he saw nothing.

"What's going on!? Who are you?! Why can't I see?!" He shouted, ignoring the fact that he can shout at all.

"Shhh. Don't get so worked up. You're safe here." The deep voice assured.

Just as he was about to retort that he felt anything but safe right now, a light appeared. floating only a few feet away, light blue eyes stood out amongst the darkness. Soon small speckles of light and luminous dots appeared, creating a clear picture of a person, a sea creature, all in one.

A mermaid.

Around him, bioluminescent algae began to glow, bright blues and greens illuminating the once-dark space to reveal the being before him. Their tail was inky black, blue and white glowing spots trailing down their long tail. All across the rather masculine-looking, pale, human chest was small spots, not unlike freckles, that also glowed. Eren was stunned.

A merman? Here? What-

The mer clicked his tongue, bringing Eren's attention to his face.

And by god was a magnificent face.

He was worryingly pale as if he never really saw the sun. The merman had sharp features, a strong chiseled jaw, and a small pointed nose. His lips were near non-existent, slightly showing sharp teeth beneath. Black hair floated around his head, the inky tendrils swaying with his movements. The blue eyes he saw earlier weren't his eyes, but another glow spot that sat where Eren's eyebrows would be. Beaneth were his real eyes. Silver irises surrounded by black sclera. He looked horrifying in his beauty, the natural allure that was famed for bringing sailors to their deaths.

The mer allowed for the inspection, black hands and arms coming up as if to reach for him. Eren let out an odd throaty noise when he saw the sharp claws coming closer to his face. He noted that the mer had some webbing in between each finger, but also that on his left hand, his pointer and middle finger had the nails completely rounded down. It was an odd detail that distracted Eren enough that he barely reacted when those hands were suddenly touching his face. The mer was close now, face only a foot away.

"Hello pretty." The dark mer greeted, an odd look that Eren couldn't place burning in his eyes.

"Who- what is going on?" The mer smirked, leaning forward and nuzzling into Eren's jaw. What.

"I've been waiting for you to awaken." Scarred hands wrapped around the side of his neck, feeling his pulse jump as he recognized how close he was to the mythical creature.

"I… How am I not dead? I was drowning." Eren pointed out.

"Nn, I saw your face and decided to keep you." He said simply as if talking about something trivial.

"What. keep me? Who are you? Where are we? How am I breathing? How am I talking?" His brain was spit-firing questions now that he realized he wasn't actively dying. The mer chuckled, moving closer much to Eren's confusion. Suddenly he felt teeth at his ear, gently nipping the lobe.

"I am Levi, and I am yours. You are mine to keep. I have marked you inside and out, as you have me. You are home." He whispered, a sudden tongue licking around his ear and down his neck. Eren felt his distress growing, confusion only growing when he tried to kick out but an odd feeling overtook him. Something isn't right.

"Yours? I- I don't understand! Why-my legs? They feel… weird." He managed to enunciate, earning a soft grunt from Levi.

"Look down." Levi murmured softly, backing away so that Eren felt he could breathe. Well, until he looked down. Where he expected to see his long tan legs, scarred from working the docks his whole life, he saw instead a long green tail. He screamed. Wildly he thrashed about, trying to get away from the shiny thing that was clearly eating his legs.

"Oi!" "IT'S EATING ME! GET IT AWAY! FuCk!" He cried out, unable to control his movements as the powerful tail propelled him this way and that. Levi managed to pin him down, holding him roughly by the neck.

"Calm yourself. Tch, you're gonna get hurt." Levi admonished, squeezing a little when Eren continued to whimper.

"It's-it's gonna- m-my legs…" Eren tried to speak, he really did, but the confusion, fear, and adrenaline were making it hard for him to speak. Levi hummed, hand roaming down his chest.

"You no longer have legs. That's your tail." Levi stated calmly, leaning in closer now that Eren was no longer struggling.

My tail? My tail? I don't have a tail. I'm a human! Not… what the fuck.

"Mmm so pretty. What are these?" Levi murmured, fingers pinching his nipple. Eren gasped, trying to push Levi away from there but he didn't budge.

"Ah stop! That's-"

"Mine. It's mine to do with as I please." Levi quickly growled, head moving up to stare into green eyes.

"Do you understand? You are alive because I kept you. You are mine." Levi's eyes were glued to his, and he could barely move beneath the heavy stare.

His?

"I-"

"What's your name?" A command.

"Eren." He whispered, too confused at the sudden situation to put up much of a fight. Levi let out a low groan, silver eyes flicking down.

"Eren. Poor little thing. I heard it can be tough for some humans when they first turn. Don't worry, I will take full responsibility. But first…" He trailed off, slowly making his way down Eren's body. Eren didn't want to look. Didn't want to see the shimmering green he knew to be there.

"I still don't understand!"

"Then let me show you in a different way~" Levi purred, and before Eren could guess what way that would be, fingers were tracing the seam of a hole he very much did not realize he had. His head whipped up so fast that his long brown hair ended up crowding his face. He quickly pushed the strands away, eyes widening at the sight before him.

A long slit sat right in the middle of his lap, where his human dick would still be had this odd tail not suddenly appeared. The lips were slightly turned outward, almost looking inflamed with how puffy they were. Levi was leering at the sight, left hand tracing the slit and teasingly dipping inside.

"This pretty little pussy was made for me. The way you tighten around my cock… nnn…" Levi looked into Eren's wide eyes.

"I haven't been able to hold back."

"Wai-" Eren was cut off by a moan when those fingers slid all the way in, reaching into spots he didn't know existed. The next few minutes were filled with Eren's gasping moans as Levi fingerfucked his newly formed puss within an inch of his life. Levi's fast pace alongside the ache of being used led to an amazing feeling running up his spine, an odd push coming from his new genitals.

"Pretty." Levi murmured, tongue laving at the penis slowly pushing out of the folds. Eren was dying for a second time. That's the only way he could describe it.

"L-Le-vi-i!" Eren stuttered out at the pace of his fingers. He felt the scrape of fingernails against his inner walls, the way he pulsed around them as he came with a shout. He spasmed against the cave wall, in a bed of soft algae that seemed to glow brighter. He was panting, hands grasping for anything around him.

Levi still suckled at his dick, the pink appendage spurting small drops now as he was sucked into overstimulation. He reached down, trying to pull Levi away from the nearing painful feeling. His hands were quickly pushed to the side and held still as Levi stared into his eyes. Eren came a second time, shouting out every expletive he knew.

"P-please! I…"

"This dick? Mine. This cunt? Mine. This womb? Mine. You belong to me, Eren."

Eren didn't have time to react when suddenly something large was pushing against the slit. He shuddered, green eyes looking down. Levi's cock was huge. Probably about the size of his arm, a dark purple color with blue splotches towards the base. It was tapered, looking a lot like a thick tongue as it speared into him. What's worse, it fit. Eren watched in horror as the entirety of the nearly 2-foot-long cock slid into him, Levi growling above him as he savagely thrust into his aching pussy. His cock

It was a tight fit, the wriggling member touching places Eren was very sure he never had before. The cock felt like it was rearranging his guts, poking at the tan of his belly and making a bulge appear beneath his skin. He laid a hand on it, surprised and horrified at the bump he felt. Levi groaned, leaning down and nipping at his cheek.

"Like that? You're so fucking tight. This perfect fucking pussy, made to house my cock." Eren moaned at the imagery, groin heating up and his mind leaving him with every thrust against him.

"I didn't believe Erwin when he said human pussy was divine. Fuck. You get it right? When a siren kisses a human… nng keep clenching me like that… fuck… when we kiss a human we turn them into mermaids. Every mermaid was once a human, every siren is the pup of a mermaid and siren." Levi revealed, leaning down to lick at his neck, biting down hard every once in a while to leave a mark. Eren could barely understand what he was saying, so focused on the huge cock hammering his insides. Every shift sent a shot of pleasure right up his spine.

"You're made to be my breeding bitch. You're my own personal cum dump. Fuck, no more ruts by myself, I get to fuck you silly whenever I want." Each filthy word that left his mouth made Eren moan, eyes crossed from the sheer pleasure that wracked his body. Levi didn't let up for even a moment, licking down Eren's neck until he found his nipple, suckling at his tit with ferocity. His right hand was wrapped around the pretty human's throat, squeezing when he pleased just to hear Eren gasp. His left hand went down to his cock, fingering the hole in between each thrust before wrapping harshly around Eren's pretty dick.

"Cum for me. Mark my skin as I breed you."

"Levi!" "Cum!"

Eren couldn't resist, the feel of lips on his nipple, that fat prick messing up his insides while Levi's hand stroked him off. He came hard, screeching until Levi pressed hard enough on his throat that no sound came out. A few more harsh thrusts and Levi came too, groaning as he emptied himself out inside the colorful mermaid. Eren cried out, feeling so full as his stomach began to bulge under the pressure.

Eventually Levi started to grind into him, Eren's pussy sucking him in despite the rough fucking he just received. Within a few minutes he was ready to go again, Eren squealing and crying as he was brought to orgasm three more times.

They lay there, Levi gently bathing his mate with his tongue, the tired mermaid simply basking in the attention of the purring siren behind him. He didn't really know what the future was, or how he ended up in this situation. But… feeling Levi's warm body on top of his, tongue gently stroking his skin, hands rubbing sore muscles with the utmost care, he figures he could get used to it. He squirmed, moaning when the cock still inside him twisted with the motion, pushing further into his filled womb.

Or maybe he wouldn't. But one thing was for sure

He'd never feel, or be, empty ever again.

So a couple of things
1. I LOVE deep sea Levi. Love it. I always envision that deep sea mers would have similar coloring to orcas. Black backs and appendages or at least a dark blue in order to not be seen by prey from above. White underbellies to blend in with the light of the surface when looked at from below. The bioluminescence of the deep ocean made an appearance since predators down there seem to enjoy attracting prey with light.
2. Eren was out for a couple of days, so as soon as that pussy started to form Levi began fucking him, to ensure it would grow to hold his cock and his alone. I tried to infer that Levi had been using his body for a while at that point but I'm not sure if that was clear.
3. Levi, despite needing his nails to hunt, decided to cut two of them down to ensure he does't hurt Eren when he fucks him. Another nod to Levi making sure Eren doesn't remain empty for more than a few moments (he's addicted to that pussy)

anyways, I hope you enjoyed and hopefully I'll see y'all on twitter tomorrow!
daemonic_nomad