After writing I remembered that Derek is not actually an Alpha after the Nogitsune. So please ignore that little deviation from actual canon for the purposes of this little snippet! .
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Stiles brings his bat down with a sickening wet thump on the creature's skull, and Derek bristles with pride at the human's vehemence. The boy is wolf in every way except the physical; a snarling, vicious thing full of fierce loyalty and unfathomable rage. The Alpha loves it, and the tiny voice trembling in the back of his mind telling him this isn't right is deftly pushed aside.
Stiles has not been the same since the Nogitsune. Everyone knows this, and the pack remains on eggshells around the human, their horror poorly hidden at this dark version of the formally carefree boy. But Derek knows the truth. He knows it's only an improvement; the thing to make Stiles worthy of the word his wolf howled relentlessly within him. Mate.
And so, after the fight has ended, and the rest of the pack are slinking off to lick their wounds; the Alpha decides he's waited long enough to take what he wants. He stalks over to the younger man and slams him into the side of his own jeep. The boy's eyes flash with surprise for just a moment before it is quickly snuffed out. This new Stiles does not like to show any weakness. The human smirks and bares his neck for the werewolf, and Derek latches on all sharp teeth and claws gripping tight. Stiles moans and rolls his body up into the older man. Derek turns his attention to that mouth he has so many twisted fantasies about, and kisses Stiles without a hint of trying to be chaste. One of his fangs' slips into Stiles lower lip, and he momentarily pauses to try rein himself in – but Stiles is only moaning louder, pushing his body harder into Derek's, so the Alpha forgoes his control and instead indulges in the hot rush of blood in his mouth.
Derek moves Stiles so that the human is facing away from him, scratched and bloody palms splayed against his car. The werewolf yanks the boy's pants down without preamble, and grasping the globes of Stiles' ass, he plunges his tongue past the soft puckered rim. Stiles lets out an almost animalistic growl, and a predatory rumble answers unbidden from Derek's throat.
Even in the midst of lust Derek knows that using his own claws would cause unnecessary damage to the boy, so he grabs the human's hand and tugs it back to position his fingers at his own entrance. Stiles catches on quickly as Derek moves forward to suck those fingers into his mouth for a moment. The younger man yanks his hand back and pushes one finger into himself. Barely seconds later he is pushing a second in and beginning to scissor himself open. Derek can taste sharp pain in the air but it is drowning under Stiles' arousal, and he faintly acknowledges he's glad the boy has made it to four fingers before he shoves the hand aside and pushes in his own aching arousal. The boy cries out and the scent of pain returns even thicker, but the human is pushing himself back onto Derek's cock and the Alpha can barely think coherently enough to hold himself back. They both need this – this fast, violent acceptance of their mutual attraction and dependency – but Stiles certainly doesn't need to be literally torn apart.
Derek powers into Stiles, taking the younger man's moans and gasps as direction and adjusting his angle until Stiles is screaming his name and digging his nails into the older man's arms with all the cut of a young wolf. With a hoarse 'fuck, Derek!', Stiles is coming in hot streaks across the car, his muscles clenching around Derek, and the werewolf follows him with roar and a bite to the neck as he fills Stiles up.
They stay like this for a moment, before Derek pulls out and Stiles lets out a low hiss before pulling his jeans back around his waist. The boy turns around and offers Derek a lazy grin before leaning forward and claiming his lips in another biting kiss. Then he leans back to finger the bite on his neck. "This won't turn me, will it?" His voice is steady, flat with mild curiosity. Stiles only sounds alive when he's killing or fucking.
"No." Derek returns, and leaves it at that. It's a claiming bite, and Stiles is clever enough to figure that out on his own. The younger male simply nods, and goes to unlock his car. Suddenly he pauses, and then is pulling Derek back in towards him by the front of his shirt. "Only fair that I should get to leave my own, right?"Derek barely opens his mouth to answer before the human is biting hard into the juncture of his neck, and the Alpha swears he can feel grins at him again, teeth red and pupils blown so wide his eyes almost look black. "Mine." The human growls, and Derek stumbles back a step as his wolf cowers and metaphorically bares its stomach.
As the clattering of the jeeps engine fades from the preserve, the werewolf allows himself a small, private smile. Man and wolf rumble simultaneously in satisfaction at the dark, twisted creature Stiles has become.