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Atsumu knows they shouldn't rest here. He felt a chill down his spine the moment he stepped into this cave, but his two adventuring companions are exhausted and adamant that they set up camp to get some food and rest.
It takes Atsumu ages to fall asleep, convinced he's seeing movement at the edge of the flickering ring of light from their small fire. When he eventually passes out, his sleep is uneasy and full of strange dreams.
He startles awake to a strange sensation covering his body and soft moans echoing in the small cave. He barely has enough time to blink the sleep from his eyes before his trousers and underwear are pulled off in one swift movement.
"H-hey!" he protests, attempting to sit up but finding his hands restrained and held by something above his head. He looks around, confusion slowly seeping into his sleepy brain, and what he sees makes his breath catch and his cock jump.
Kiyoomi and Rin are on their knees, their hands bound with what looks like slightly transparent green tentacles. Both of them are panting and rolling their hips into a waist-high mound resting on the ground between them, its colour the same transparent green as the tentacles. Even Atsumu's sleep-addled mind can immediately deduce that this is a slime creature, and he remembers what they feed on. If this thing has them captured, they're going to be here for a while.
His cock jumps again at the thought, starting to harden properly as warm slime wraps around him and lifts him up into the same kneeling position as his friends. The green mound quivers slightly before him, and a tentacle snakes around his hips to wrap around his half-hard cock. It strokes him steadily and with purpose, and Atsumu can't help but moan as pleasure floods his pliant body.
"A-atsumu," Kiyoomi stutters out, lifting his head and looking at him with lidded eyes. "Atsu, fuck, we-"
"Just give it what it wants," Atsumu gasps, cursing as the slime tentacle skillfully works his cock to full hardness. "It won't - oh fuck - it won't hurt us."
A broken moan to Atsumu's right makes him turn his head, seeing Rin's hips stutter out of rhythm as he comes into the creature. His spurting cock is visible through the transparent slime, throbbing and twitching as the creature greedily feeds on his cum. Atsumu swallows, feeling heat rush through his body at the sight.
"Ah, oh my god," Rin pants, slumping a little in the tentacle's hold as he rides out his orgasm. "I thought- oh shit! "
A small mass of slime crawls up behind Rin, wrapping itself around his hips and attaching itself flush to his ass. Rin's eyes suddenly shoot wide open and he throws his head back, a high-pitched shout echoing off the cave walls.
"It's in m-my ass," Rin groans, his hips jumping again as the mound clenches around his cock. "Holy shit, holy fucking shit."
Atsumu opens his mouth to say something, anything, but is interrupted by his own surprised groan as a tiny tentacle, barely thicker than a piece of string, starts to pay attention to his sensitive tip. It moves slowly, spreading precome around, before it prods at his slit. Atsumu's breath catches and he feels a split second of fear before the tentacle slides in, and oh. Oh fuck. It's unlike anything Atsumu's ever felt, but he already knows he could become addicted to it.
The tentacle goes slow, sliding down the inside of his cock and stopping every once in a while to swirl around, making him buck and whimper with need. When it reaches as deep as it can do it pulls out an inch or two before pushing back in, slowly fucking his slit and driving Atsumu up the wall with confusing pleasure.
He looks up at his friends - Rin who is fully hard again and fucking the creature with renewed vigour, and Kiyoomi who's still going slow but looks like he's going to come any second now - and feels a pinch of desperate envy. He wants to be inside the creature as well, wants to feel those soft, warm walls surround his aching cock and make him scream out his release.
It doesn't help when Kiyoomi chokes out a moan as he comes, the mound shaking as he drives his hips harder with each spurt of cum. Atsumu watches him go, his brain swimming with need but helpless to do anything about it.
"Please," he breathes when Kiyoomi also gets a smaller slime behind him, his brows drawing up in pleasure as the creature slowly pushes into his ass. "Lemme fuck ya, let me come, god, please."
The tentacles around him twitch and pulse, and suddenly there's a pressure on his back as he's being pushed forward into the mound. He almost cries with relief when his cock breaches the slime's outer membrane and becomes enveloped in tight heat. It's about as warm as a human but much softer and slicker, the mass tightening around his cock even as the smaller tentacle is still happily pumping his slit. It feels strange yet incredible and Atsumu can feel every trace of rational thought slip away from him. All that's left is cock, hole, come.
It doesn't take long for him to reach his first peak, and Rin reaches his second one about the same time. Atsumu pants and bucks as he finally climaxes, his cum seeping out around the small tentacle in his dick. Rin just groans softly, whimpering slightly when the creature keeps working him.
We're nowhere near done yet, Atsumu thinks with a shiver as he feels something breach his ass and head straight for his prostate, making him curse and his cock twitch with renewed energy right after his orgasm.
To his left Kiyoomi comes again, with a whimper and a soft sigh as he feeds the slime his cum which it happily accepts. Atsumu closes his eyes and lets himself be utterly and thoroughly used.
It goes on for hours, maybe more, Atsumu stops keeping track. Eventually they all pass out, one by one, and when they wake up the cave is clean and empty. Their fire is still going and their clothes are back on.
It's as if nothing strange even happened.