The first time it happens, it's unplanned and Alec is completely unaware. As Jace passes by his parabatai's room on his way to the kitchen, he stops immediately when hearing what sounds like a painful groan, coming from behind the door. With just one step he's there, palm already on the handle, when he hears the same sound again and it makes him change his mind; now that he thinks about it, that groan doesn't really sound to be caused by pain, rather by pleasure.
For some reason, Jace stands there mesmerized as his brain puts together the puzzle pieces, and he gasps when he realises what exactly is going on behind the closed door. His breathing turns heavier second after second, and it isn't long before he is panting softly with his eyes closed, forehead rested against the wooden surface as he listens closely, leaving the rest up to his imagination - though that soon turns out to be nowhere near enough.
His fingers slowly slip from the handle and slide into the pocket of his pants, pulling his stele out, and he draws the rune by heart, not even having to look there. The rune glows for a short second or two before taking effect, causing a square-shaped hole to appear on the door, exactly at Jace's eye level, and that finally makes him open his eyes, peeking inside. He knows it's wrong, he knows he shouldn't, but the sight and the sounds combined make him unable to move, let alone leave, so he just watches almost motionlessly, licking his lips.
Of course, he has seen Alec naked before, since they have lived together at the Institute for most of their lives, but the last time was months ago, maybe even a year or more, and Jace is now surprised by how toned and muscular Alec's body is. He is obviously strong, the runes varying in colour from the fresh, bright black to old, faded white on his skin, which is pale and makes his scars stand out more. If Jace was just a little less confident, he probably would be jealous.
He watches Alec's face for what seems like forever but actually isn't more than five or six seconds; he watches the closed eyes, watches as his parabatai's lips part and he lets out that same sound again, watches as Alec throws his head back, his back arching, upper body lifting off of the bed just slightly. Jace's glance then slowly moves along Alec's arm, down all the way to his fingers that are wrapped around his cock, stroking himself at a quick, obviously impatient pace.
Jace doesn't want to react, but his brain seems to be unable to control his body anymore, his skin becoming warm, erection growing under the fabric of his clothes, and he lets out a tiny, shaky sigh. It's so silent that it's nearly impossible to hear, but it's enough to make him come to his senses, drawing the opposite rune of the previous one with a shaky hand, and rushing away from that room.
What he doesn't realise is that, no matter how small he drew them to be, the runes left a mark on the surface of the door, just like they do on the skin.
xxxx
The second time is unplanned again, but Alec is already suspicious. He's a Shadowhunter, so he easily noticed the marks on his door when he left the room after the first time, the marks he could have sworn weren't yet there when he entered the room barely more than half an hour earlier. There was a mix of feelings battling inside as he thought one by one of the reasons why the rune marks might be on his door (there aren't many options), and the people who might have made them (there are quite a few). And even though he would never admit, there were a handful of those options that managed to make him frustrated, either in one or both meanings of the word.
Still, all of those worries are forgotten only a few days later as Alec sits naked on the bathroom's floor, legs pulled up and parted almost painfully wide, his hand sliding slowly down on his own upper body, before starting to tease his length with tender brushes of his fingertips, letting out a long, shaky breath. He has drawn a locking rune on the door, then another one just to make sure, so he feels safe and free of any worries.
He moves a little, just enough to be able to rest his back against the side of the bathtub, his eyes closing as he shivers from the cold touch of the porcelain against his back. It manages to calm him down a little, but it still isn't long before he has to bite his lower lip to stay silent, his free hand's fingers running slowly through the dark locks of his hair. His other hand is palming his own cock teasingly slowly, causing his body to tremble from the pleasure.
It builds up bit by bit, and it feels like it takes nearly forever - just the way Alec loves it. The palming turns into stroking and it evolves into hard pumping, but he stops several times right when he thinks he can't take it anymore. He's driving himself crazy, testing his own limits, and he knows the exact point where he reaches them, the moment when the pleasure simply becomes too much, and he finally pushes himself over the edge with a few more powerful strokes, a loud moan falling from his lips.
For a few minutes he just sits there, regaining his energy and waiting for his breath to turn even again, for his body to stop trembling. When he finally manages to convince himself that he needs to get up and really needs a shower, he slowly opens his eyes, letting them get used to the bright light of the bathroom's lamps. As he stands up and is about to turn towards the shower cabin, he notices something that makes his heart race nervously: the bathroom's door is unlocked and slightly open.
He walks over with slow steps, opening the door further but he doesn't see anyone. He glances at the outer side of the door, finding the marks of the unlocking runes that deactivated his locking ones, and if possible, his heart starts beating even faster than before. He knows more than well that there are only two people who can deactivate a Shadowhunter's rune: one is said Shadowhunter themself, and the other...
Their parabatai.
Jace...
The realisation makes Alec blush and smirk all at the same time.
xxxx
The third time is planned carefully, but is also supposed to seem accidental. Jace is out on a mission with three or four other Shadowhunters: Alec can't recall the exact amount or who the others are, since he failed to pay attention to most of what has been said earlier. It's some everyday thing, might as well be a false alarm, surely nothing they couldn't take care of; and nothing that would take longer than an hour, Alec guesses.
Entering Jace's room is a piece of cake - and for once Alec doesn't even feel guilty about deactivating his parabatai's locking rune, since Jace did the same with his one only a couple of days ago. He takes his time walking around in the room, smiling at how everything is a little chaotic, but Jace still finds peace and comfort in it. Alec has always been the type of person who keeps changing things until they are perfect in his point of view, and Jace the type who changes his point of view until things seem perfect. And Alec can't stop getting amazed by that over and over again.
As much as he feels tempted to put Jace's books into alphabetical order and to fix one of his parabatai's jackets that is hanging lopsidedly, he reminds himself how Jace also hates having his things touched, and how his plan could end horribly even without extra details to upset Jace, so he decides not to touch or re-organize anything.
Instead, he walks over to the bed and slowly sits down on it, kicking his boots off and lifting his feet up to the sheets as well. Within seconds his shirt lands next to the boots on the floor, and Alec lays down, smiling at the softness of the blanket under him and the way Jace's scent embraces him immediately. Even that is already enough to make him excited, but for now he simply waits, counting the seconds and the minutes, and paying attention to the parabatai rune and bond, to know about when to expect Jace's return.
It takes twenty-three minutes before Alec decides it's time, almost completely sure that Jace has arrived back at the Institute and will be back in his room after a shower. He slowly, patiently rids himself of the rest of his clothes, until he is laying completely naked in Jace's bed. His eyes close, a playful grin appearing on his lips as he hears the water splashing in the bathroom right next to Jace's room, and even though he is slightly nervous, his excitement easily wins.
By the time the sounds of the water die down, Alec has already managed to turn himself on almost painfully, but he keeps his strokes slow, not wanting to reach his orgasm too soon. He pretends not to hear or notice when Jace enters the room, even though his parabatai's presence is obvious, and simply continues with his plan. Knowing that Jace most probably watches him and having the younger man's scent surround him is doing wonders to his body, tricking his brain into believing that those fingers on his cock are actually Jace's, instead of his own.
Until they truly are replaced by Jace's fingers a minute or two later, causing Alec's body to arch immediately, his hand withdrawing and moving to grip onto the sheets instead. Jace is either nervous or clumsy, or maybe both, but Alec still moans from the touches as they drag him dangerously close to his limits, faster than his own strokes ever did. He is panting heavily, chest rising and falling rapidly as his muscles tense, and his fingernails dig into the fistfuls of the bedsheets he's gripping. The sound he lets out is a mix of a raw, deep groan and a little scream, the sticky evidence of his orgasm escaping between Jace's fingers and spurting onto his own abdomen.
Alec feels the room spinning slightly, his whole body numb from the overwhelming amount of pleasure, his lips swollen from the way he bit them earlier, and parted as he's still panting. He can feel Jace's fingertips tenderly caressing his length the whole time as he is trying to calm down, and once he finally opens his eyes, he finds himself face to face with a smirking, obviously proud Jace. There's silence between them, none of them knowing how to break it without sounding awkward, but it's not uncomfortable, and Alec soon reaches to the nightstand next to Jace's bed, grabbing a tissue. He then takes his parabatai's hand, cleaning it slowly, carefully, glancing back and forth between Jace's face and hand several times.
Once he is finished Jace leans forward, and for a second Alec thinks he is going to kiss him. Instead, Jace's lips brush softly against Alec's ear as he whispers a few words to him, still smirking. "The next time you enter my room without permission, I'm killing you." The playful tone he uses makes Alec grin as well, and Jace bites his earlobe as he pulls back, getting up to go and find some clothes to replace the towel around his waist with.