Host Trails - Luis Serra x Reader (Beneath the Rot Part 5)
Summary: You are both called in to do live host implantation trials. It doesn't go particularly well. During the process, Luis realizes some of the reasons he likes you so much.
Content Warning: Human experimentation, some gore, some angst
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The request comes early. Too early, judging by the way the messenger outside Luis’ lab door avoids his eyes while delivering it.
“Dr. Serra,” the man says stiffly, holding out a clipped folder stamped with one of Saddler’s sigils. “You’re needed downstairs.”
Luis takes the folder without touching the man’s hand. “How thrilling. Do I at least get to know why?”
The messenger’s expression barely shifts. “Host trials.”
Fuck. Of course.
Luis has already felt the familiar tension settle beneath his ribs before he even opens the folder. He flips through the first few pages with a glance, jaw tightening at the sight of the schedule, the names, the dosage adjustments, the bloodwork requests.
And then he sees your name on the list of needed employees. His fingers still. For half a second, he just stares at it.
Then he closes the folder with a sharp, quiet snap.
He finds you ten minutes later, near the adjoining workstation, bent over a tray of slides with your hair pinned up messily and your sleeves already rolled to the elbows. You look up when he approaches. At first, you smile at the sight of him. Your face drops when you see his expression.
“What’s that face for?” you ask, eyes narrowing faintly. “You look like someone died.”
Luis gives you a smile that he knows is too thin to be convincing. “How observant of you.”
That earns him a suspicious look.
He lowers his voice. “We’ve been called downstairs.”
Your brows lift. “For what?”
He hesitates. Not long. Just long enough for the answer to become obvious. Your expression changes first. Not fear exactly. Just a careful stillness, the kind that arrives before trouble does.
“... What kind of thing are we doing downstairs?” you ask quietly.
Luis leans one shoulder against the counter beside you, keeping his voice light even as his stomach turns. “The kind that involves clipboards, steel tables, and the sort of men who call suffering progress.”
You stare at him for a long moment.
Then your lips flatten. “...I’m assuming that is not a good sign.”
“Ah.” He huffs a dry little breath. “You are learning so quickly.”
You do not smile. That alone tells him more than he wants to know about how serious this is. He watches your fingers tighten around the pen in your hand.
“We have to assist? With the host trails?” you ask.
“Yes.”
Your gaze flickers downward. “And the hosts?”
Luis doesn’t answer immediately. That is answer enough by itself. Your face goes very still. He wishes, abruptly and with unexpected violence, that he could take the morning back. To repeat everything with the added insistence that you were home with a nasty flu. Instead, he reaches for your wrist before he can overthink it. Not hard. Just enough to get your attention.
“Hey,” he says quietly. “Stay with me.”
You blink once, sharply. Then nod. Too quickly. Your face steels, “... Let’s go.”
.
.
.
The host trials take place three levels beneath the main laboratory, far away from the cleaner research wings shown to new recruits. Down here, the air smells faintly of antiseptic failing to cover rot. Rows of reinforced observation rooms line the corridor behind thick glass panels, each occupied by villagers in varying stages of infection. Some are restrained for their own safety. Others simply sit motionless on metal cots, heads tilted faintly as though listening to something no one else can hear. The purpose of the trials is deceptively clinical on paper: monitor parasite adaptation within live hosts over extended periods of time. Measure aggression. Obedience. Cognitive decline. Physical mutation. In reality, it’s watching human beings disappear piece by piece while researchers record the process in neat handwriting.
Luis exhales through his nose, adjusting the collar of his lab coat as he rounds the final corner. The room beyond the observation window is already active. Researchers in white coats and cultists in dark robes move between the metal tables with the brisk efficiency of people who have learned how to look unconcerned in the presence of suffering.
He hates how normal they make it look.
And then he sees you.
You’re standing just inside the threshold, one hand still wrapped around the strap of your messenger bag, the other holding a pair of fresh gloves that you have not yet put on. You look as though you are trying very hard to remain composed. Too hard.
Luis notices the stiffness in your shoulders before you see him.
That doesn’t surprise him.
What does surprise him is how quickly he finds himself paying attention to nothing else. You look up when he steps closer, and for a moment your expression clears in the relief of seeing a familiar face. Then your eyes drift past him, silently toward the room beyond. The relief vanishes almost immediately. He follows your gaze.
Several restrained hosts lie on the tables beneath the lights. Their bindings are secure. Too secure. The kind of security that only ever gets used when someone expects the human body to fight back. Thick straps across wrists and ankles. Metal supports at the neck and shoulders. Monitoring equipment already attached. One of the hosts is awake.
He watches the man’s chest rise and fall in short, strained breaths.
No one here calls them patients. No one here calls them people either. To the cult, they are little more than livestock. It makes him have to suppress a bubble of nausea in his throat.
“Dr. Serra,” one of the supervisors says, clipboard in hand. The man barely looks at him before glancing toward you, still standing beside him. “Your toxicologist is needed to help with the secondary monitoring.”
Luis’s jaw tightens faintly. He does not miss the way your fingers curl just slightly around the gloves in your hand.
“She’s new,” He smiles, tone light enough to be polite and sharp enough to warn. “You could at least pretend to ease her into this.”
The supervisor gives him a thin, unreadable look. “We are not here to coddle anyone,” he grunts.
Luis almost laughs. Instead, he steps aside and lets you move forward. You hesitate only once before entering the room.
That, more than anything, makes something unpleasant twist in his stomach.
.
.
.
The lower chamber is colder than the lab above. Colder, and worse. The air down here smells different; antiseptic layered over old sweat, blood, metal, and the damp earth beneath the facility. The lights are harsher too, unblinking fluorescent bars that flatten everything into pale colorless geometry.
Luis hates it. He hates the way the room is laid out with clinical precision, as if that makes what happens here less obscene. He hates the steel tables and the thick leather restraints. There’s trays of stainless steel instruments. Observation windows. Charts already prepared for failure rates and response times.
A line of guards stands near the far wall.
A man in a lab coat Luis recognizes from upstairs is speaking in a low, excited tone to one of Saddler’s followers. The same words in a different accent. A different kind of fanaticism.
You do not speak at all when they escort the first host into the room. He feels your body go rigid beside him.
The man on the table looks like he might once have been a farmer. Or a father. Or both. His wrists are strapped down, his shirt half-open to allow access to the torso beneath. He is breathing too quickly. Sweat clings to his forehead.
His eyes dart around the room wildly. Not for answers. For an escape. For an end to it all.
Luis looks away for half a second and immediately regrets it. Because when he looks back, the host is speaking. Not loudly. Not clearly. Just pleading.
It takes Luis a moment to realize the words are in Spanish. The man is begging. It makes his throat tighten. Beside him, you have gone entirely still. You probably understand enough to know exactly what is being said. Your mouth parts faintly, but no sound comes out.
One of the researchers mutters something about stability thresholds and compatibility markers. Another checks a chart. Someone in the back of the room scribbles notes as if they are not all standing in a chamber built around the slow dismantling of a human body.
Luis has to force himself to keep his face calm. He has done this before. That is the ugly truth of it. He has stood in rooms like this before and told himself the same lies; that he was observing, that he was helping, that if he stayed close enough to the process, he might stop it from becoming worse.
And then he sees your face. Suddenly, all of this feels uglier than usual.
The host cries out when the first incision is made. Not from pain, exactly. Not exclusively, at least. It’s from fear. Raw and human in a way that makes his throat feel tight.
His voice cracks on the second attempt to speak. Luis hears his own name in it nowhere, but hears the same thing anyway. The same helpless pleading. The same animal terror of being trapped while something foreign is placed inside him.
Your fingers twitch once at your side. You do not step away. Luis notices that too. God help him, he notices everything.
The trial begins with all the brittle confidence of men who think cruelty is the same as control.
It is not.
The parasite reacts faster than expected. Too fast. The host spasms violently against the restraints, body arching hard enough to rattle the table. One of the assistants reaches for the sedative dose. Another shouts for the data recorder to increase the reading.
Luis is already watching you. Not the host. You.
Your expression is tight, pale under the cold light. But you are not looking away. Not even when the host starts crying out again, voice breaking on the edges of panic and pain.
You go very, very still. Then, quietly, you ask the question no one else in the room seems interested in hearing. “Does he know what’s happening to him?”
No one answers at first. Luis turns his head just enough to look at you properly.
The lead researcher scoffs. “Of course he knows. He is the host.”
You inhale once. Luis can almost feel the shift in your breathing before you speak again.
“That wasn’t my question.”
The silence that follows is sharp enough to cut. Luis feels the pulse in his jaw.
You are trying so hard to remain professional that it is nearly painful to watch. Your hands are clenched at your sides. Your shoulders are rigid. But you do not leave. You don't even flinch when the host convulses again.
That, more than anything, hits him where it hurts.
He steps closer to you on instinct, just enough that his shoulder brushes yours. The contact is brief. Enough.
“Corazón,” he says under his breath, “if you need to step out, do it now.”
Your eyes stay fixed on the table. “I’m fine.”
He almost laughs. The word is so obviously false it is almost offensive. But you don't move.
The trial continues.
There is a moment , a terrible, suspended moment , when the host’s body goes shockingly still. For one wild second, the room seems to inhale with it. Then the parasite responds beneath the skin, and the man jerks so hard against the restraints that one wrist splits open against the metal. Dark, almost brownish blood oozes slowly onto the floor.
The trial continues with the kind of clinical detachment that always makes it worse.
Readings. Notes. Temperature checks. Exposure adjustments. All the polite little words that let men pretend they are not doing violence.
Luis positions himself near the observation monitors, pen in hand, while you are directed toward the far side of the room to assist with the equipment. You do so immediately, but he notices the delay before your gloves go on. One second. Two. A tiny hesitation at the wrist before you force yourself to keep moving.
Not fear exactly. Something sharper. Something more dangerous. Empathy.
The host thrashes once as the first stage of post-insertion stimulation begins. The room tightens around the sound. Luis keeps his face neutral.
He has done this before. Too many times. Enough times to know that looking disgusted only earns suspicion, and looking indifferent only earns promotion.
Still, he catches the way your hands tremble ever so slightly.
You are assigned to monitor the pulse and observe response patterns on the auxiliary screens. Simple work, technically. The kind that can be done without ever looking directly at the host if one is careful enough.
He can tell immediately that you’re trying not to look. As it always does with new researchers, that fails.
The first time, your gaze slips toward the restrained man on the table when he makes a small, broken sound in the back of his throat. Your eyes meet his before darting away, almost too scared to acknowledge him directly.
The second time is worse. He watches your head turn despite yourself, as if something in you refuses to let the moment go unheard. Refuses to disregard the suffering, even if it's for the sake of science.
Luis looks at you instead of the monitors for a little too long. Your face has gone pale beneath the overhead lights.
The parasite response increases. One of the supervisors keeps turning up a little dial on one of the numerous machines. A cultist reads the numbers off the screen in a flat voice while the host begins to shake against the restraints. Not violently. Not yet. Just enough to make the straps creak. The man on the table says something in Spanish. He catches it instantly.
“Por favor. Ayúdame!”
The word hangs in the room like a blade. He sees you flinch. Not dramatic. Not enough for anyone else to notice. But he sees it. Your jaw tightens afterward, and your eyes lower immediately to the clipboard in front of you. You begin writing. Too fast. The pen nearly slips once, and when it does, you grip it harder.
He has the strange, sudden urge to take it from you. Or to stop everything entirely. Neither is an option.
“Readings?” the supervisor asks.
You force herself to respond, voice steady only because you are clearly fighting for it. “Elevated heart rate. Increased motor response. Neural instability is worsening after stimulation.”
Luis hears the slight fracture on the last word. He looks back at the host.
The man is breathing hard now, head turned to the side, eyes squeezed shut as if that might make any of this less real. The parasite beneath the skin shifts in faint, disturbing motion.
Luis knows what is coming next. He also knows that you do not. Not fully.
When the second stage begins, your face changes. He watches it happen in pieces. Confusion first. Then realization. Then horror so immediate and complete that it seems to drain the color from your expression all at once.
He knows you’ve realized it now.
You’ve realized that whatever they have introduced into the host is not meant to help him survive the process. It is meant to force compliance.
His body tenses before his mind does.
The host jerks sharply against the restraints, eyes flying open, panic overtaking the empty, drugged stare from moments before. The sound he makes is awful; half pain, half terror, all human.
One of the cultists says something dismissive in the background. Luis does not bother listening. His attention is fixed on you, standing beside the console. Your hand has frozen over the notes.
You’re staring at the table now, face tight with a kind of contained revulsion that makes you look heartbreakingly young for a second, and for one brief, ugly moment, Luis hates this room more than he has hated it in years.
He sees the way your throat moves when you swallow. He sees the way your eyes flick toward the host and then away again, as if you cannot decide whether looking is an act of cruelty or refusing to look is.
The answer, he thinks, is worse than that.
The host starts speaking through clenched teeth. Begging.
Luis understands the words to wish he didn’t. It almost makes him hate his native tongue.
Your shoulders draw inward sharply, the instinctive reaction of someone trying to make themselves smaller inside their own skin. And still you keep writing. Still you keep checking the pulse monitor. Still, you keep working.
That is what nearly breaks something in him.
Even through the horror, the realizations, the disgust, you keep working like this is any other day.
He knows what is coming next. He wishes he could warn you. But he can’t. Instead, he looks over, meeting your eyes for a brief second and praying that you understand what he means.
This is going to be bad.
Your lips thin again, looking back down to your clipboard like that will make this all stop. God, he wishes it would all stop.
Of course, it doesn’t. The third stage begins with a needle. Small. Clear. Almost harmless looking.
Luis feels your attention lock onto it immediately.
One of the supervisors says something about ‘compliance acceleration,’ voice bored in the way only monsters can manage. Another researcher adjusts the restraints again while the host breathes in short, ragged gasps through clenched teeth.
“No,” the man whispers suddenly. Not loud. Not dramatic. Just terrified. “No, no, por favor-!”
The injection goes in anyway. The reaction is immediate.
The host’s entire body seizes violently against the restraints hard enough to shake the table beneath him. A scream tears out of him before he can stop it, raw and animal and agonizingly human. Monitoring equipment spikes all at once. One of the assistants curses. Another starts recording numbers faster.
Luis barely hears any of it.
Because you flinch like you’ve been struck.
Not outwardly enough for anyone else to care. But he sees the way your breath catches. The way your hand braces suddenly against the edge of the console to steady yourself. And still… Still you keep working.
“Heart rate spiking,” you manage quietly, staring fixedly at the monitor instead of the man. “Neural activity increasing across-” Your voice almost breaks. You stop. Swallow hard. Continue anyway. “- across all monitored regions.”
The host keeps screaming.
It changes after a while. Becomes rougher. Wet at the edges. Like his throat can’t physically withstand it anymore. Luis very suddenly hates every person in this room.
The parasite moves visibly beneath the man’s skin now, writhing along his neck in sickening little pulses. One of the cultists murmurs something reverent under his breath at the sight of it.
He wants to put a bullet through his skull.
Instead, he forces himself to keep standing there. Keeps writing notes. Keeps pretending his hands are not curling hard enough around the pen to ache.
Beside him, you suddenly move. Not away. Closer.
You step toward the table before you can seem to stop yourself, eyes fixed on the host’s face. The panic in his expression is fading now, blurring strangely at the edges beneath whatever cocktail they forced into his system.
“Can he hear us?” you ask quietly.
The room barely pauses.
One of the supervisors shrugs. “Irrelevant.”
Luis sees something in your face crack at that. Tiny. Almost invisible. But final. Your jaw tightens sharply before you step back again, retreating toward the monitors like you hate yourself for asking in the first place.
The trial continues another twenty-three minutes. Luis counts them. Every single one.
By the end, the host is barely responsive. His breathing has gone shallow. Blood streaks the restraints around one wrist where he fought hard enough to tear skin open against the metal. The parasite activity stabilizes eventually. The researchers seem pleased by that.
The man on the table does not.
“Record final observations,” the supervisor orders.
The room shifts immediately back into motion. Pens scratching. Equipment shutting down. Calm conversation returning like none of this mattered at all.
Luis looks toward you. You are staring at the clipboard in your hands like you can’t understand what it is anymore.
“Dr. L/N?” someone prompts sharply.
Your shoulders jerk faintly. Then, mechanically, you begin reading the data aloud.
Luis has never heard your voice sound so empty.
.
.
.
The hallway outside the lower chamber feels too narrow. Too bright. Too warm after the freezing air downstairs, even though the temperature is only a few degrees higher.
The heavy security door slams shut behind both of you with a metallic thud that echoes down the corridor. For a moment, neither of you speaks. Researchers pass occasionally through the halls carrying charts and specimen cases, completely indifferent to the fact that there is still blood beneath Luis’s fingernails.
You walk three more steps before stopping abruptly.
He turns towards you like its instinct.
Fuck. Your hands are shaking. It’s not subtle anymore. It’s violent. You stare down at them like they belong to someone else entirely. “Oh.”
It’s barely a whisper. That single word nearly destroys him.
The realization is hitting you all at once now that the room is gone. No monitors. No supervisors. No pressure forcing you upright. Just silence and fluorescent lights and the aftermath of understanding exactly what this place really is.
“Oh my god,” you breathe again, quieter this time.
Luis moves before thinking. His hands catch your forearms gently just as your knees threaten to buckle. You suck in a sharp breath like the contact alone almost undoes you.
“Easy,” he murmurs quickly. “Easy, hermosa. I’ve got you.”
Your face twists suddenly.
“I didn’t do anything,” you whisper. The horror in your voice is unbearable. “I just stood there.”
Luis feels something ugly split open in his chest.
“No.” His grip tightens slightly. “No, this isn’t on you. You couldn’t have done anything to stop that.”
“That’s not better.” Your voice cracks hard on the last word.
For one awful second, he thinks you might actually collapse. Instead, you press the heel of your hand hard against your mouth like you’re physically trying to hold yourself together. Your shoulders shake once. Twice.
Luis glances down the hallway automatically before pulling you closer against him, shielding you partly from view.
“Hey,” he says softly. “Look at me.”
You can’t. Your eyes stay fixed somewhere near the floor, wet and horrified and distant all at once. “I should’ve stopped it,” you whisper. “I should’ve said something. I should’ve-!”
“And then what?” Luis asks quietly.
The words come out sharper than intended. You finally look up at him. His expression softens immediately afterward.
“They would have removed you from the room,” he says more gently. “Or worse. And that man would still be on that table.”
Your face crumples slightly. “That doesn’t make it okay.”
“No,” he admits.
His thumb brushes unconsciously against your sleeve where his hand still steadies your arm. “It doesn’t.”
The silence afterward feels enormous. Luis studies your expression for a long moment before exhaling quietly through his nose. “Listen to me carefully,” he says.
Your eyes lift toward him again.
“The fact that this upset you means there is still something human left in you.” His jaw tightens faintly. “Down here, that matters more than you realize.”
For a second, you just stare at him. Then, your eyes squeeze shut. Luis pulls you against his chest before you can argue with him about it this time.
And unlike before, this time, you don’t resist at all.
