As usual, Lisa checks in on him before she leaves.
"Jean told me you turned down her offer for dinner," she says by way of greeting, with a frown that looks both disappointed and pitying at once. "Do you really plan to spend your birthday alone in the dark, Sir Kaeya?"
Kaeya could argue that he isn't in the dark; after all, while the sun has long since set, a lantern still burns gently atop the table, the light by which he requires in order to continue his work. He could also argue that Lisa's own presence disproves her claim that he is currently "alone", as they are both the last remaining occupants of the library.
However, Kaeya is well-aware of the consequences suffered by those who draw the librarian's ire, and in consideration of his own physical well-being, wisely chooses against picking a fight over semantics with Lisa Minci this time. (It is his birthday, after all.)
"Oh, you know how it is," says Kaeya, setting his research to the side before looking up to send Lisa an affable smile. "Crime never takes a day off, and neither can I."
"Not even on your birthday?"
"Not even on my birthday."
"You and Jean can be terribly similar sometimes," says Lisa with a deep sigh of resignation, and then she reaches in her bag, pulls something out, and sets it in front of him.
"Is it just me, or are wine bottles getting smaller?" asks Kaeya, picking the container up and examining the liquid inside.
"It's a calming draught," says Lisa, ignoring Kaeya's comment, as she rightly should. "One of my own brews, and very potent. Take just one spoonful an hour before bed, and it should help."
Kaeya finds himself at a momentary loss for words by how much the gesture touches him. Lisa is one of the rare few who knows about his nightmares; he'd been a little too honest over drinks in Jean's office one evening months ago, and he's frankly surprised she even remembers.
"Thank you," he says sincerely, setting the bottle down and looking up at her.
Lisa's smile is unflinchingly kind, and not for the first time Kaeya wonders if he deserves it. "You can thank me properly by coming out for dinner tomorrow. No excuses this time, all right?"
She only leaves after Kaeya promises to be there, and he waits for her footsteps up the stairs and out into the hall to fade before rising from his seat, lantern in hand to guide his way towards one of the back corners of the library.
It's been another long evening attempting to decipher one of the coded messages an Abyss Mage had been careless enough to leave behind following an infiltration of one of their hideouts. The leader of the Abyss Order seems to enjoy speaking of a Tevyat that dates back hundreds of years, often referencing villages and natural landmarks which no longer exist, or recalling events that can only be found in pieces across various travel journals. While Kaeya has been pursuing the Abyss Order long enough to familiarize himself with the basic structure of the cryptic missives, not even a mind as clever as his can avoid putting in the legwork required to peruse thousands of lines of (often illegible) text.
Would he rather be at dinner with Jean and Lisa, sipping on smooth dandelion wine and dodging pointed questions about certain complex relationships in his life? Absolutely. But Kaeya has never found it particularly easy to relax when there is unfinished work to be done, especially when it involves an organization he has a personal interest in dismantling.
Kaeya is on one knee, examining the dusty titles on a lower shelf, when the lantern he'd set on the floor suddenly flickers out and sends his surroundings into near-complete darkness. The only sources of light that remain are the thin rays of moonlight coming in through the tall windows and the pale blue glow of his Vision at his side. It's an unpleasant surprise; he'd been certain that there was enough oil in the lamp to last another few hours yet, but perhaps he'd been mistaken.
It happens all too quickly—Kaeya moves to retrieve the lantern, but with his guard down and his sight yet to fully adjust to the shadows, his reflexes are slow, and he's unable to defend against the sudden grab of an unknown assailant. They grapple for a short time, but the attacker has strength and momentum on their side, and it isn't long before Kaeya finds his cheek pressed against the wall with both wrists behind his back, caught in a loop of chains.
"Good evening, Captain," murmurs a quiet voice in his ear that sounds unsettlingly familiar.
"Forgive me if I am unable to address you by your proper title in kind, considering how you have yet to introduce yourself," says Kaeya, who is probably speaking too many words for one still struggling to recover his breath from the force of the shove and resulting impact. "But good evening to you as well. May I assist you with anything?"
"Do exactly as I say," says his assailant, sharp and authoritative, and despite everything Kaeya cannot help but think that if they keep using that voice, he'd be tempted to do nearly anything demanded of him. "Walk to that chair over there and sit down with your arms out. No tricks."
Curious enough to play along, Kaeya follows the orders without complaint, and a moment later his arms are folded behind the chair back, wrists tied together with a length of rope that is much kinder to his skin than the chains, but still holds fast when he gives it a subtle tug. Next, a thick blindfold descends over his eyes, effectively removing any chance he has of being able to visually identify his captor.
"Dare I ask why you've left me free to speak?" asks Kaeya. "I could have shouted for help by now, you know."
"You could have," his captor agrees, "if there was anyone left in the building to hear you. But there isn't, is there?"
There isn't. Even Albedo and Sucrose, who tend to keep odd hours with their work in the lab, are out of the city on a research expedition. This is objectively a terrible night for Kaeya to have been caught alone, unsuspecting and unarmed.
"So you see, Captain," continues his captor, voice close to his ear again, "I am the only one here to listen to your cries for help—as well as any other sounds you happen to make."
With how much care had gone into the way his restraints had been tied, Kaeya had begun to have his suspicions, but now that the stranger's intentions are making themselves clear, his heart begins to beat faster in his chest. He barely dares to breathe as deft hands find their way to his waist, where they begin to undo the fastenings on his trousers.
"If it's my body you want, surely we can negotiate some terms to satisfy us both," says Kaeya to the pitch-black space ahead of him, deliberately testing his captor's patience. "I can't say I've never fantasized about being manhandled by an attractive stranger in the library, though I suppose it's rather difficult for me to tell exactly how attractive you are beyond what I can glean from the pitch and timbre of your voice, which I do find very plea—"
Three gloved fingers force their way into Kaeya's mouth, effectively cutting him off. They taste of leather and iron and—most significantly—Pyro elemental energy. It is the final clue that allows Kaeya to identify precisely who he is dealing with now.
"Spare me your tricks, Sir Kaeya," says his captor calmly, and the fingers in Kaeya's mouth push in deeper still, until his jaw is forced open wide enough to hurt and he has to resist gagging. "You may not have your sword, but your tongue is notorious for being twice as deadly. I have questions for you, and should you choose to withhold the answers I seek, I will do what must be done to test your resolve. Understood?"
The fingers pull out so he can answer, and Kaeya immediately coughs and spits out the excess saliva that had gathered in his mouth on the floor.
"Do your worst, Darknight Hero," is all he says, and then he smiles, which is clearly the wrong thing to do, because it earns him a sharp slap across the face that leaves his cheek stinging.
"You doubt my intention to follow through," says his captor, still speaking calmly in unsettling contrast to the strength behind the slap. Kaeya badly wishes he weren't blindfolded, so he could look properly into the eyes of the vigilante his men have been hunting down for over a year. "Very well. We'll start with a demonstration."
Kaeya finds himself all too sensitive to the cool air brushing between his thighs as his trousers and smallclothes are pulled down as far as they'll go past his knees, exposing him fully. He isn't aware of how hard he is until he's taken roughly into a gloved hand, partly lubricated from his own saliva but not enough to eliminate the friction from the texture of the leather. He bites down on his own tongue to resist providing any sort of desirable reaction, but when the hand gives the base of his cock a tight squeeze he can't help but suck in a low hiss through his teeth.
"Fine," says Kaeya, taking slow, deep breaths, and the alleviation of the pressure between his legs is not enough to prevent the dread that settles in his stomach. "What do you need?"
"The Fatui. I know you visited the Goth Grand Hotel this past week. What did you discuss with them?"
Kaeya knows his answer isn't going to go over well. "Unfortunately, I can't tell you that."
A very deliberate pull, and Kaeya is unable to stifle a low grunt.
"Care to repeat that?"
"Can't tell you. It's classified information," says Kaeya, with a strained, breathless chuckle. "Knights only, I'm afraid. You're free to apply in the spring."
After a pause, his captor removes his hand entirely, allowing Kaeya an unexpected but not unappreciated moment of respite. There's a quiet slide of fabric—gloves being removed, he thinks, and his assumption is proved correct a second later when distinctly bare fingers grab him roughly by the chin, forcing his head up and his jaw open.
"You piss me off," murmurs his captor. "Arrogant. Annoying. You always find something to be cocky about, don't you, Captain?"
Kaeya senses the rise in temperature in the air around them before he senses anything else, and then there's a flare of searing pain on the skin at the side of his neck. He only manages to stop himself from crying out by tensing his jaw and keeping his lips clamped shut until the heat fades, but he has no doubt that it's left its mark.
"Do you know how easy it would be for me to burn a hole through your throat so you can no longer speak?" his captor asks, and despite everything, the back of Kaeya's neck prickles with interest and morbid curiosity. "Fortunately for you, I still need information, so here's what's going to happen. I'm going to fuck you until you're begging me to let you come." The words accompany the unmistakable sound of fabric and belts dropping to the floor. "Then I'm going to ask you again what your plans are with the Fatui."
"And if I don't answer?"
It's the last thing the Darknight Hero promises before he moves in to stand over Kaeya's lap, and Kaeya's breath hitches at the touch of cool metal against his flushed skin—some sort of circular contraption that slides down and then locks to a perfect fit around the base of his cock.
"Oh, this is the fancy type of sexy interrogation," murmurs Kaeya in a comment that appears to go ignored as he's grabbed and held in place. He expects his captor to need another moment to prepare, but in the next second Kaeya feels himself push through only minimal resistance before becoming enveloped in heat.
"Do you prepare yourself this well for every knight you question or am I special?" Kaeya cannot help but ask, even as he bottoms out and feels the other clench tightly around him, making his words come out in a half-moan.
He hears a sound that is somewhere between a huff and a sigh, and then a chest pressing against his own as his captor leans forward to speak into his ear.
"Can you stop trying to make this more difficult than it already is?"
"Sorry," Kaeya whispers, fighting the urge to grin. "I'll be good."
"Of course you will," his captor mutters, and then talking time is clearly over when he braces his hands on Kaeya's shoulders and begins to move.
It's slow, at first—agonizingly slow. Kaeya is a patient man, but even he has his limits, and when he attempts to lift his hips to take some semblance of control over the pace he's punished with another lick of fire against his neck that makes him hiss. He learns his lesson after that and doesn't dare move again, channelling all of his restless energy into his tightly clenched fists behind the chair back.
"Imagine if your knights could see you now," says his captor, still speaking directly into his ear. "Do you think they would do anything to help you? Or do you think they would just stand there and watch?" With nothing to see except the vivid mental images that come to mind, Kaeya makes a sound deep in his throat, desperate and wanting. "You would enjoy that, wouldn't you? You would enjoy being held down and used in front of all of your men. Everyone could take a turn with their beloved captain. Is that what you want? Answer me."
"Yes," says Kaeya, without missing a beat.
"Whore," says his captor derisively, and then he grinds hard into his lap, and Kaeya almost comes right then—probably would have, were it not for the contraption still locked around him, which he's become acutely aware of the more he swells and strains against its hold. "Unfortunately for them, your cock belongs to me. You belong to me. Say it."
"I belong to you," says Kaeya, breathless.
"So you can be good," murmurs his captor with an approving hum. "Very well, Sir Kaeya. Perhaps a taste of positive reinforcement will motivate you to cooperate."
Finally, finally, his captor begins to ride him in earnest, until their once-quiet corner of the library is filled with the obscene sounds of skin slapping against skin and wood creaking beneath their shared weight with every movement. A short distance away, there's a soft thump and a rustle of paper, and then a louder clatter of metal and glass; for a heartstopping moment, Kaeya wonders if they've been caught, but no footsteps follow and he's relieved (and admittedly, a little disappointed).
Then he hears his own name carried on the whisper of a breath behind his ear, and all idle thoughts of a thrilling encounter with a potential voyeur disappear.
"Kaeya," he hears again, in a soft plea that is completely unlike the rough and demanding tone of his assailant.
If it was only that, perhaps Kaeya could have kept up the act. But then there are hands sliding down his back and pulling their bodies closer together, and he finds his face pressed against the dip of a warm neck, draped with loose strands of hair that tickle his nose. Instinctively, he breathes in, and his lungs fill with the combined scent of lampgrass and campfire smoke he's been addicted to for as long as he can remember, and Kaeya snaps.
He freezes his restraints until they crack and shatter, rips off his blindfold, grabs Diluc by the hips and fucks him, earning a cry of surprise that Kaeya catches with his mouth and turns into a muffled moan. The kiss is harsh and hungry and desperate, and Diluc is helpless to stop the choked noise his throat emits every time he falls on Kaeya's cock just right, and Kaeya swallows each one of them up like a man starved and then chases after more—
—and more, until the chair beneath them creaks loudly in distress, and Kaeya makes the strategic decision to slide his hands under Diluc's thighs and pick him up just long enough to stumble over to the closest table (miraculously, without tripping over the trousers still looped around his ankles) and drop him down on the surface.
At last, they break apart for air, and Kaeya realizes he can actually see Diluc now, not only because he's now free of the blindfold, but because they've moved out of the shadows and into the moonlight.
"You're wearing your mask," says Kaeya, once enough oxygen has returned to his brain.
"Yes, it's part of the disguise," says Diluc, as though Kaeya's an idiot, which they both know isn't true but is well-deserved all the same.
Kaeya reaches up with both hands to take the mask off, which Diluc allows with no protest.
"Aha," he says. "I knew it."
"Congratulations," says Diluc dryly, plucking the mask from his hands and dropping it to the floor.
"You could sound a little cuter," says Kaeya with a small grin, hands now free to return to Diluc's face and cup his jaw on both sides. "It is my birthday, after all."
Diluc closes his eyes, no doubt to summon a great amount of patience, but Kaeya chooses to take it as a cue to kiss him again, slowly and sweetly this time. When he pulls away, Diluc's eyes are half-open and gazing back at him with something indiscernible.
"Now, where were we?" Kaeya murmurs, dropping his hands down to Diluc's thighs and pushing them apart.
Just like that, the mood shifts, and it doesn't take either of them long to get back to where they were before the impromptu intermission. They lose the rest of their clothes, and Diluc ends up on his back, hair loose and wild and fanned out beneath his head. He's always gorgeous, but like this, with his body fully bared and laid out across the table under the light of the moon, he's a work of art; a work of art Kaeya could spend the entire night taking apart to admire until he's memorized every beautiful inch with his hands and his mouth alike.
Unfortunately, tonight they don't have all night. In fact, they've most likely overstayed their (dubious) welcome as it is; Albedo and Sucrose could return from their expedition at any time, Lisa could come back to check on Kaeya, Jean could fetch something from her office and hear suspicious noises coming from across the hall, Klee could—oh, no. Klee is absolutely not allowed to walk in and see any of this.
And with that grim reminder of their risky circumstances, Kaeya grabs Diluc's legs and throws them over his shoulders, nearly bending him in half as he works with renewed urgency. A high-pitched whine escapes Diluc's lips, which he would normally get absurdly embarrassed about (no matter how many times Kaeya tells him it's cute, which probably makes it worse), but they've been at this long enough that he's clearly abandoned any pretense of pride, which is Kaeya's absolute favourite part, because this is when he gets to hear things like—
"Don't stop. Don't you dare stop. Please. Kaeya. Please. Please, please, please."
There is not a single thing in all of Tevyat hotter than Diluc Ragnvindr beneath you, begging to come on your cock.
Kaeya does not dare stop until he feels Diluc clench and tense around him, before finishing all over his own chest and collapsing as much as he can with his legs still hooked over Kaeya's shoulders. He breathes heavily, eyes open but vacant, and Kaeya presses a kiss to the side of Diluc's leg before resting his head against it and gazing down at Diluc's face thoughtfully.
"You're staring," mutters Diluc once he's mostly recovered, meeting Kaeya's eye briefly before looking away again. He's not wrong, so Kaeya doesn't have an argument for that, but before he can come up with some kind of reasonable excuse, Diluc suddenly frowns and shifts his hips in a way that definitely gets Kaeya paying attention to his own body again. "You haven't...?"
Oh. He hasn't.
"The ring," says Diluc slowly, as though suddenly remembering something.
"The ring?" repeats Kaeya, utterly baffled, until his genius brain catches up and puts the pieces together. He pulls out of Diluc, looks down, and—there it is. The ring. The metal contraption locked securely around his dick that he had somehow forgotten about, just like he'd somehow forgotten about coming at all, because he'd been too preoccupied with staring at Diluc.
Diluc, who sits up with an apology on his tongue, but before he can reach out and help Kaeya take off the ring, a voice calls out from the floor above them.
Jean's voice, to be specific.
"Kaeya? Lisa mentioned you were staying late, and I thought you might... Hm? Why is it so dark?"
The ring doesn't come off until much later, and the sheer relief its removal brings is almost enough to make up for the thoroughly humbling scolding Jean had given the both of them.
"I'm happy for you two," she'd said at the end, exhausted and pink in the face. "But please keep it out of headquarters."
(And then she'd made them clean up, of course.)
"Where did you get this?" Kaeya asks, holding the metal up curiously.
"Albedo," says Diluc, and Kaeya laughs, until he realizes Diluc is serious. Albedo? Albedo makes sex toys? There is a story here and Kaeya doesn't think he'll be able to sleep until he hears it. But before he can ask, Diluc speaks up again. "Listen, Kaeya, I wanted to—"
"Don't even think about apologizing," Kaeya cuts in. "The whole scenario was my idea. We may have gone slightly off-script, and your fun little gadget here was a very interesting surprise, but I do take full responsibility for what happened, including getting caught."
"I'm not apologizing," says Diluc. "It was your idea."
"Oh," says Kaeya, taken aback. "Then?"
Diluc cracks a smile, and the sight almost takes Kaeya's breath away. Here, lying in bed together in their shared room in their shared house with the gentle morning light peeking in through the windows, Kaeya is vividly reminded of simpler times, when Diluc's smiles were wide and bright and handed out for free. They were precious to Kaeya then, and they're precious now, no matter how rare they might be. (Though, he thinks with a heart that feels impossibly light, they might be a little less rare these days.)
In the next moment, Diluc's smile touches his own for a brief, sweet second.
"Happy birthday," he whispers.
It might be the best one Kaeya has ever had.