Part 5 of the Ruined series.

The silvery lines on Stiles' arms shimmer in the sunlight that pours in through the loft windows. Derek watches from the floor, back against the wall in the corner where the bed used to be, as Stiles and Lydia seem to face off against one another.

The sun makes Lydia's hair glow golden. There's a red scarf around her neck, and when he squints, Derek thinks it looks like blood on her throat.

He's starting to think like Stiles, to see things as he might.

Sweat beads at Stiles' temples, on his arms. He's still too thin, no amount of coaxing can get him to eat some days. "What if it explodes?" he says.

"It won't," Lydia says. "Because if it does I'll kill you. It was expensive."

"What if you explode?" Stiles' hands are restless. They clench into fists and then splay wide, over and over again.

"I trust you." Lydia lifts her chin, swallows. "Do it."

Stiles closes his eyes, breathes long and slow like he's exhausted. He opens his eyes, his breathing still labored, his head tipped slightly to one side.

The scarf around Lydia's throat pulls tight, the fabric spreading out as it goes taut. Lydia's eyes go wide, she sucks air into her lungs and lifts a hand to her throat.

Her hand stops half-way, and she forces it, fist clenched tight, back down to her hip. "Okay," she says, voice calm and measured, but Derek can hear her heart and it's pounding hard and fast.

Stiles lets out a whimper, and the scarf tightens around Lydia's throat. She tips her head back, lifting her chin, stretching herself out to her full height and Derek doesn't know if she's doing that or if she's being pulled up. He gets his hands and feet beneath him, ready to cross the room to stop what's happening when her heels hit the floor and the scarf unravels, flicks up into the air, and flutters down.

Derek stops, frozen half-way to his feet as Stiles takes two steps forward, hands outstretched, fingers shaking as he reaches for Lydia. He wraps a hand around her throat, thumb stroking over her windpipe and Lydia's heart is hammering like a rabbit's.

"Stiles," Lydia whispers.

Derek pulls himself up. "Stiles." His own voice is a warning.

Stiles' head whips around, looking at Derek as though he'd forgotten he was there. "She's bleeding," he says. He pulls his hand away from her throat and doesn't seem to notice as she goes stumbling back. He lifts his hand, shows it to Derek, palm forward. "Blood." His eyebrows are scrunched, like he's confused, like he can't understand why Derek can't see it too.

Derek shakes his head. "There's no blood, Stiles. Look again."

Stiles turns his hand, holds it close to his face. He closes his eyes, opens them and blinks in confusion. "I don't—"

"I'm fine," Lydia says, stepping forward, taking his hand by the wrist. "I'm okay, Stiles."

Stiles stares down at her. "I'm sorry." He lifts his hand to her cheek, then he looks up, over to the scarf where it lies on the floor in a puddle of red. He reaches out with his free hand, and the scarf streaks through the air and lands in his palm. "I got confused," he says. "You should have worn another color."

When Lydia's gone, Stiles slides to the floor under the duct, leans into Derek's side. "I can do it," he says. "But it's too risky. She's so pretty in red, I didn't want it to stop." He turns his head, looks up at Derek from beneath long, wet, black eyelashes. "I wanted to watch it flow, wanted to watch her choke on her own blood." He drags a finger down the line of his throat, then spreads his fingers out over his heart. "It's crawled in here. I can taste it." He drags his tongue over his teeth. "Tastes like gasoline. It shows me what it wants me to do."

"What does it want you to do?" Derek whispers. He asks, now, asks before Stiles tells him in great detail anyway.

Stiles shifts, gets his leg over, settles in Derek's lap. He yanks at the buttons of Derek's shirt until they pop open, then presses his thumbnail into the indentation at the base of his throat. "I start here," he says, and drags his thumb down, all the way to Derek's belly. "It used to give me a knife, but I don't need it now." He drops his head, places a kiss so gentle and soft to Derek's chest. "I peel back your flesh, crack open your rib cage." His lips move like a caress over Derek's skin. "Hold your heart in my hand while it's still beating." He starts to shift against Derek, to rub against him through their jeans.

Derek gets hard in spite of himself, in spite of the fear that sits heavy in his gut. "I'll die if you do that, Stiles," he says, grabbing Stiles by the back of the neck, hauling him up as he wraps his other arm around Stiles' waist and pulls him hard against him. "You know that, right?"

Stiles nods. He closes his eyes and smiles. "Only reason I haven't done it yet." His eyes snap open. "It feels like it belongs there now. I'm never going to get rid of it, you know? I'm never going to stop wanting to kill my friends."

"Use it," Derek says. He kisses Stiles, sweetly, a mere brush of lips. "Use it to fight the bad things."

Stiles' eyes are wide and trusting. "What if I am one of the bad things?"

"I don't know," Derek says. "I really don't know."