They fuck first, because Derek says the scent will help keep him anchored. Stiles isn't sure that's true, but he pushes Derek down on the bed and rides him until he comes all over Derek's stomach and Derek is spilling deep inside him.
They go out to the old Hale house and Stiles makes a circle of Mountain Ash, Derek standing naked in the center. Once it's closed Derek starts pacing from one side to the other, eyebrows drawn in concentration. Stiles watches, quiet. He's worried Derek won't be able to do this, but also worried he will.
When the shift finally begins Stiles stops breathing. It's both amazing and terrifying; Derek's body shakes and twists and grows twice his normal size, black fur covering pale skin. Derek's eyes glow red, his face contorting into a wolf-like snout, sharp fangs snapping dangerously.
Stiles can't look away as Derek drops to all fours and paces the edge of the ash circle, powerful muscles shifting beneath his fur. He stops when he reaches Stiles, staring at him with blood red eyes. Derek's Alpha form looks nothing like a regular wolf, huge and frightening and the most beautiful thing Stiles has ever seen.
He reaches out slowly and Derek huffs, nosing at his hand when it crosses the ash. Stiles draws in a sharp breath, fingers carding through silky fur, and Derek leans into his hand before stepping away. Stiles doesn't think about what he's doing, just follows him, stepping into the circle and away from the assurance of safety.
Derek growls and Stiles freezes, heart racing, but then Derek is reaching out and carefully taking the open edge of Stiles' plaid shirt in his teeth, tugging him further in. When they reach the center Derek leans against Stiles, pushing his weight into him and making him go sprawling on the leaf-covered ground. Stiles stares up at Derek with wide eyes, more confused than anything else.
Until Derek starts nosing at the crotch of his jeans, rubbing his snout along Stiles' soft dick. Stiles lets out a strangled noise and tries to scramble back, but Derek presses one huge paw to his chest, holding him in place. He rubs his nose harder against Stiles' dick and Stiles flushes, feeling himself start to grow hard. He swallows; this wasn't part of the plan, but oh god, fuck, he wants it.
"Okay," he says, toeing off his shoes and slowly reaching for the button on his fly. "Just let me – I want to –"
Derek moves back and watches as Stiles undresses, now achingly hard. As soon as he's naked Derek steps forward again and licks over Stiles' dick with his tongue, making Stiles gasp and buck his hips. It's soft and amazing, and Stiles groans, wondering if he'll ever be able to see Derek's Alpha form after this without getting turned on.
"Is this what you wanted?" Stiles asks when his dick gives a hard twitch under Derek's tongue. "Because I'm about to come, and I don't know if you – I mean, I'm okay with more –"
Derek huffs and pulls back. Stiles feels a spike of disappointment, but then Derek nudges insistently at his hip and Stiles moans, immediately rolling over and lifting his ass in the air, spreading his knees. Derek moves behind him, snuffling at where Stiles' hole is still open and wet from earlier. His paws come down on either side of Stiles' head, huge body covering Stiles completely, his dick rubbing over Stiles' ass, and oh god, that's bigger in this form too.
Stiles makes himself relax and just keep breathing as Derek pushes in, stretching him wider than he's ever been stretched before, filling him so full he thinks he might burst. It hurts but it also feels really fucking good, and his lips part on a whimper. Soft growls are echoing from above him, and Stiles is pretty sure his own dick is hard enough to cut diamonds, leaking pre-come onto the ground below him.
When Derek is finally all the way in he pauses. He's stretching Stiles so wide Stiles' legs are trembling, body trying to clench down around the huge intrusion, and Stiles can barely believe it all fits. Stiles moans, and Derek must take that as permission because he starts snapping his hips, hard.
Stiles cries out, arms collapsing at the force. He is literally being fucked into the ground by a huge-ass werewolf, and it's the hottest thing that has ever happened to him. Derek's thrusts are wild and abandoned, animalistic, his claws digging into the dirt next to Stiles' face, and all Stiles can do is clutch at his hair and fucking take it. He makes choked-off noises, eyes tightly closed, and he's not going to be able to walk properly for days.
Stiles almost stops breathing when he feels Derek's dick start to grow even bigger right at the base, tugging at Stiles' rim with each pull out, and he's not sure why he didn't think this might happen. Derek has knotted him before, not every time but often enough, and Stiles groans with nearly overwhelming pleasure as he thinks about being tied to Derek like this.
Derek slams inside one last time, shoving the still-growing knot in as hard as he can, and Stiles shouts a surprised curse as he comes. His body shakes, dick jerking as he coats the ground in strips of white. It's intense, his stretched out hole clenching around Derek's huge dick, knot pressing against his prostate and making his vision blur.
Above him, Derek pants loudly, and Stiles can feel every pulse of his dick as he fills him with hot come. The thought makes him shiver with renewed arousal; there' s going to be so much come everywhere when Derek is able to pull out. That won't be for a while, though, so for now Stiles shifts, trying to lie down flat, and Derek carefully moves with him.
"I love you," Stiles says, words slurred, and he's never said that before but he means it, oh god, he means it.
Derek whines, the sound happy, and snuffles along Stiles' jaw until he can lick at Stiles' mouth. Stiles laughs giddily and reaches up to scratch his fingers through Derek's fur, body thrumming with the afterglow, enjoying the feel of Derek locked inside of him.
Eventually, Derek's knot shrinks enough that he can pull out. A gush of come follows him, wetting Stiles' balls and thighs, and Stiles whimpers when he feels the soft lap of Derek's tongue against him. Derek pulls back and Stiles rolls over, watching as he shifts back into his human form.
"Are you okay?" Derek asks, voice rough and eyebrows drawn together.
"I'm perfect," Stiles says with a grin, stretching. His ass twinges and he hums, reaching out for Derek. "Come here."
Derek curls around him, arm tight around Stiles' waist and nose pressed against his neck. "I didn't mean for that to happen."
Stiles bites his bottom lip, fingers tangling in Derek's hair. "Do you regret it?"
"No," Derek says, fierce.
"Then stop worrying, because I don't either." Stiles tugs at Derek's hair until he looks up at him. "It was fucking amazing, and something I would love to do again. You know, after my ass recovers. If you want to."
Derek studies his face, hand sliding up to cover Stiles' steady heart. "I do."
"Good," Stiles says. "Now help me up so we can go back to your loft and sleep on a real bed."
Derek huffs out a laugh, and kisses Stiles until they're both breathless.