Derek made it easy for him, holding still and keeping quiet while Stiles concentrated on tying the knots. The rope was more than long enough for the purpose, but Stiles felt a hot spark of accomplishment when he tightened the last loop around Derek's wrist, and he knew he'd gotten it right.
Well, he was ninety percent sure he'd gotten it right. He eyed the heavy frame of Derek's bed again, ran his fingers over the knots one last time, and then kneed Derek gently in the ribs.
"Try it," Stiles said, trying not to give anything away in his voice. The handy thing about Derek already being naked, and Stiles being almost naked, and having negotiated for kinky sex that was now in the process of happening, was that Derek couldn't read anything from his heartbeat that wasn't obvious just from looking.
Derek opened his eyes and smiled slightly--humoring Stiles, playing along with the scene they'd agreed to--and jerked his wrists against the ropes.
He frowned and did it again, harder, and Stiles sat down on Derek's thighs a second before Derek threw his whole body into it, arching off the bed even with Stiles's weight on him. The bedframe creaked but held, and the rope didn't give an inch.
Derek's face went slack with shock, recognizing that he was actually trapped, that this was no mere gesture at bondage. The semi Stiles had been sporting all the way through the setup from the mere possibility of this moment went completely hard so fast he got a little light-headed.
Derek struggled a little more, the stunned look hardening into a scowl, and the guilty-hotness of it tipped over into guilt. Stiles hadn't told Derek what he was going to do, and this wasn't about really hurting or scaring or imprisoning him. It wasn't okay to get off on it if Derek really didn't want to be tied down like this.
Stiles rocked forward, dropping both of his hands onto Derek's chest with his weight behind them. Derek could throw him off--well, he could if he weren't tied to the headboard--but it was enough force to make him stop fighting for a second.
When Derek met his eyes Stiles gave him an uncertain smile. "Yeah, so there's mountain ash soaked into the core of the rope. None in the outer layer, so you shouldn't get rope burn unless you manage to fray it, but you're not going to get loose without doing yourself and the bed some real damage. Just say the word and I'll undo it."
Derek's scowl deepened, and he jerked against the ropes but didn't fight hard enough to budge Stiles. "Fuck. You."
That definitely was not Derek's safeword. Stiles glanced up at Derek's hands to double check. He flexed his fingers open and closed, coming nowhere near snapping his fingers, which was the backup signal.
The guilt vanished in giddiness, and Stiles couldn't help grinding his dick down against Derek's, which was, he realized, just as hard. Derek was on board, and it was going to be terrifying later that Derek was letting him do this, but right now--he had Derek Hale at his mercy, really genuinely tied up and helpless to escape.
Derek growled faintly and shoved his hips up, and, God, Stiles could get off just on this, just on the fact that he had Derek tied up, that he'd gotten away with it and it had worked. He could get Derek off like this, too, from the feel of him. God knew they'd done enough adrenaline-fueled frottage; this might be a new flavor, but it wasn't exactly going out on a limb. Derek was getting into the scene, though, playing angry and fighting the ropes, and Stiles could work with that.
He pushed up and off of Derek, shoved his boxers off, and grabbed his gym bag from under the bed. He'd learned a while ago that he could get pretty much anything past Derek's nose if he wrapped it in his lacrosse gear. There were only a limited number of things he was willing to subject to that treatment, but the mountain ash switches he'd cut that morning weren't any the worse for the wear, still green and flexible.
Stiles straightened up and swished the two branches through the air, making a satisfying sharp sound. Derek's eyes widened and his breath hitched, but he just jerked his arms against the rope some more. He wasn't trying to protect himself or making any gesture toward safing out.
"You know what this is," Stiles said, even as he concentrated on the truth of it himself, believing it and making it real all at once. "Mountain ash. If a little of it in the ropes can keep you down, you know what the actual branches can do to you, right?"
Derek bucked up, his whole body bowing off the bed, a neat arc from his shoulders to his heels. Stiles set one switch down neatly on the bedside table to use later, and then smacked the other down across Derek's chest when he fell back to the bed. He scowled a little himself as he concentrated on believing he could actually do this. The rope had worked, so the mountain ash ought to give him enough leverage for this, too.
Derek's arms flexed again, trying to haul himself against the ropes, but--he didn't come off the bed. Stiles's mouth fell open as he watched the muscles in his shoulders flexing, tendons standing out in his neck and sweat sheening his skin as Derek tried, and failed, to push through the barrier of a flexible stick of wood thinner than Stiles's finger.
"Oh, fuck," Stiles said, reverently, pressing the flat of his hand against his dick. Derek turned his head to look at him and snarled, snapping his teeth, and Stiles grinned helplessly, because this was awesome.
He dragged the switch down Derek's chest all the way to his hips. In his peripheral vision Stiles saw Derek's shoulders come off the bed as he struggled against the ropes again, but all Stiles could do was stare at the way he wriggled his hips, pinned to the bed by Stiles, by Stiles using the power of the mountain ash. Derek was hard, foreskin drawn back and the head of his dick all shiny-wet with readiness, but he couldn't lift his hips where Stiles was holding him down without even using both hands.
Derek cursed breathlessly, barely more articulate than the growling, but Stiles didn't even look up, just rocked the switch back and forth, giving Derek an inch of wiggle room on the right side, then the left. He watched Derek try desperately to push up into that space before Stiles forced him back down. He watched the way Derek's cock bounced with the movement, the way it jerked even when Stiles held Derek's hips still, because, fuck, Derek was getting off on this as hard as Stiles was. Stiles closed his hand on his own dick, jacking himself a couple of times because he couldn't resist--but, no, he definitely wasn't just standing next to Derek's bed and jerking himself off when he could do something better.
He yanked back the switch and watched Derek pump his hips up into the air, sudden and uncontrolled because he'd been expecting resistance. Derek let out a wordless furious snarl at that, and Stiles didn't really think. He pivoted and swung the switch, slapping it right across Derek's open mouth. It slotted between his teeth like a bit into a horse's mouth, and Derek's snarl trailed off into a choked noise and harsh breathing. He watched Derek strain against it for a few seconds, lips and tongue working uselessly, his head forced back when Stiles pushed on the switch with his thumb. Stiles's vision was going bright-edged and his head was getting light--oh, did he have his hand on his dick again? yeah, yeah he did and he wasn't stopping--and then there was the unmistakable sharp snap of fingers.
Stiles yanked the switch back and dropped it, raising his empty hands. He took a step toward Derek to untie the knots, but Derek was already shaking his head, making a come on, come on gesture. He'd only been objecting to having his mouth stopped.
Stiles took a deep breath and grabbed the switch off the nightstand rather than take his eyes off of Derek's face. He was sex-flushed, the gray-green of his eyes nearly swallowed up in black, teeth bared. Stiles summoned up all his will and every ounce of the carefully focused intent he'd been learning to turn on and off as necessary, and he swung the switch so hard it whistled before it cracked down across Derek's chest.
He meant to say something about teaching Derek to interrupt him, something to make a point of the totally symbolic smack. But when he lifted the switch away there was a bright pink stripe across Derek's chest, and Derek made a noise suspiciously like a moan before he went back to snarling.
"Fuck me," Stiles whispered, kneeling on the edge of the bed to look. The welt wasn't disappearing. Stiles rubbed his thumb over it and Derek squirmed under the touch, his breath hitching again and again. He couldn't even growl until Stiles took his hand away.
"Oh yeah," Stiles said, remembering every bruise Derek had ever given him, every scratch, every awkward patch of stubble-burn. He hadn't actually minded any of them, but he was going to repay them all now, and Derek was going to fucking love it. "Yeah, we're doing this."
Derek growled--Stiles actually knew that one, that was the impatient stop talking and do it then growl. Stiles twisted and brought the switch down hard on an angle, across his belly and the top of his thigh. Derek arched up, letting out a high keening sound, and pre-come was beading up on the head of his cock. Stiles turned and took aim at his arms next, raised over his head as he struggled. The switch made a perfect red line across the triceps of both arms, perfectly aligned from one to the other. Derek's breath stopped sharply; he shuddered all over and then started snarling again.
Stiles laid down another stripe across his belly, and then another, changing the angle, cutting one welt across another. When he realized that Derek was sprawling on purpose, he moved between Derek's legs and with careful, precise swings he lashed the insides of Derek's thighs, and every red line that appeared on Derek's pale skin went straight to his dick.
Derek stopped fighting about then, and Stiles had an excellent view of his cock responding, his balls drawing up tight. When Stiles moved on to switch his stomach from this new angle, Derek hooked one leg around Stiles's thigh and gasped out, "Stiles, Stiles, please."
"What's that?" Stiles said, tilting his head and raising his arm again. That hadn't been a please stop, and he was curious to see if he could make Derek come just from this. "You'll tear my throat out?"
He slapped the switch down again and again, crosshatching the marks he'd already made, until Derek's low desperate please please please formed into an actual request: "Please, Stiles--please, t--touch me."
Stiles went still at that, watching Derek until Derek opened his eyes and looked at him. He wasn't fighting the ropes anymore, and he tugged gently with his heel against the back of Stiles's thigh, not even trying to force him. Stiles folded the switch into a loop and held it in his left hand, leaning forward to press his right hand down against a criss-cross of welts on Derek's stomach.
Derek arched into his palm, his cock bouncing against nothing and his eyelids shivering down. "Please."
Stiles tossed the switch away--it occurred to him vaguely that Derek might not be able to get out of the bed, depending on where the switches had landed, but that wasn't really a downside right now--and he laid his left hand down next to the right. He could feel the raised lines of the welts, slightly hotter than the rest of Derek's feverish skin. He dug in the fingertips of his right hand and dragged them down over the marks until his hand reached Derek's cock.
Derek went completely still for a second when Stiles wrapped his fingers around his cock, and then Derek groaned like it hurt in the best possible way. He pushed up into Stiles's grip, his body rippling to push up into Stiles's hand flattened on his chest. Stiles started jerking him off--nothing special, just a firm grip, slicked with the pre-come Derek had been leaking since they started--and let his other hand roam. He explored every mark he'd left on Derek, lightly tracing one with his fingertips, digging his knuckles into another, scratching another cross-wise and watching the marks of his fingernails disappear while the welt from the mountain ash remained. Derek was writhing, pushing constantly closer to Stiles, taking whatever Stiles gave him. He'd gone past words now, panting softly.
Stiles leaned over him to press his palm to the mark crossing the back of Derek's arm, and that put him nearly face-to-face with Derek.
"Hey," he said softly, tightening his grip on Derek's dick, and Derek opened his eyes. Stiles dug the heel of his hand into Derek's arm, and Derek arched up under him, his dick seeming to swell in Stiles's grip as he came, his mouth falling open without a sound. Stiles pushed back to sit on his heels, stroking Derek through it, watching come splatter down onto his belly, slickly shiny over the pattern of marks he'd left. Even before Derek finished, Stiles was rubbing come into Derek's skin, watching the way muscles jumped in his belly as he did.
Derek actually did try to pull away when his dick started going soft in Stiles's grip, and Stiles let go and moved up to kneel astride Derek's belly. He leaned down, bracing his hands on either side of Derek's head. Derek was totally limp now, nearly passed out, just a sliver of his eyes showing through the lashes. Stiles kissed him deep and slow as he rubbed his dick against the mess he'd made of Derek, the sticky heat of jizz and welts and Derek's skin. It didn't take long; Stiles had felt like he was on the verge of coming ever since he'd realized the mountain ash was working, and everything after that had been a bonus.
He groaned into Derek's mouth as his dick jerked against Derek's belly, adding his own contribution to the mess. Derek's mouth moved against his in slow motion, Derek's teeth dragging glacially against his lower lip, and Stiles lost track of everything for a little while.
When his brain reassembled he was still holding himself over Derek. He took a couple of deep breaths and then pushed himself up, grabbing the rope-end that would release all the knots with one hard tug. The knots behaved--maybe literally obeyed him, because he was definitely willing them not to have pulled annoyingly tight in all of Derek's struggles--and the rope fell away from Derek's wrists.
Derek's arms fell down onto the mattress, and Derek stayed totally still for a few seconds. Stiles looked down at him, fucked out and banged up in a way Stiles had never managed before, and then both of Derek's hands shot out. Before Stiles quite knew what had happened he'd been hauled down to the bed. Derek rolled over on top of him, pinning him thoroughly and getting him as covered in come as Derek was. Stiles figured that was only fair at this point.
Still, he knew he had responsibilities as the one who'd done the tying-up-and-beating. He worked one arm out from under Derek and set his hand gently on Derek's side. "Hey, are you good? Was that all right for you?"
"Shut up or I'll tear your throat out." Derek had his face pressed into Stiles's throat, so the sleepy monotone was somewhat muffled, but Stiles could decipher that sentence just from the cadence of noises.
"'Kay," Stiles agreed. "So, are you going to heal anytime soon?"
"Magic injuries take a little longer," Derek said, actually tilting his head to get the words out clearly. Then he pressed his mouth to Stiles's skin again, so Stiles mostly had to infer that the next thing was, "Now shut the fuck up and enjoy it."
Stiles closed his eyes and obeyed, already half-dreaming of the next time.