A/N Ok, so this is movieverse mostly, and as this is fanfiction, I have changed a lot of things…also, some of the information and facts I have come from other vampire and werewolf myths, stories and legends. Please bear with me! I hope that this isn't terrible.
Jacob can't stop his head from hurting as he comes to. He doesn't know where he is or what happened, he only knows two things: first, that he is chained to a wall, and second, his entire body is aching. This can't be good, he thinks to himself as his eyes begin to adjust to the dark room. Just as he's thinking that, it gets worse. The door opens and several people – if they're even people at all, enter. Jacob doesn't recognize any of them at first, but as they come closer, he realizes that he knows two out of the six or seven new comers. Jasper Hale and Alice Cullen are being dragged along, presumably, Jacob thinks, by the same beings that brought him here. He can't place their scent or see their faces, though, which makes him all the more nervous.
"Stop," one of the masked beings declares. The others stop, two each holding the vampires in their grasp. Jasper's hands are chained behind his back, which confuses Jacob, because he knows that Jasper is strong enough to repel chains – unless they're charmed or something. However, the means by which Jasper is restrained quickly becomes insignificant as the being speaks again. "Drink his blood." The being approaches Jasper. "Come on, Hale, bite the werewolf."
"N-no," Jasper shakes his head. "I won't do it."
"You're enemies," the being purrs. "Bite him."
"I WON'T!" Jasper roars. He knows what will happen if he bites Jacob; he knows what werewolf blood will do to him; he knows that it will make him crazy, that it will make him violent, and Jasper doesn't want to hurt anyone.
"You will," the being seethes. "Or we will kill Ms. Cullen."
"NO!" Jasper lunges at the being, struggling against the two holding him, but he can't get free, he isn't strong enough, and the pull him back, laughing at his attempts.
"Oh yes," the being nods. "Make your choice, Jasper, bite the werewolf or lose your mate."
"Fine," Jasper gives in, breathing heavily. He allows himself to be dragged forwards, closer to the wall where Jacob is chained. They push Jasper right in front of Jacob, holding him so that he can't run. Jasper looks up and immediately reads the terror in Jacob's eyes. "I-I have to," Jasper whispers.
"I know," Jacob nods. "Just do it."
"I'm sorry." Jasper lowers his head and opens his mouth. In the seconds before Jasper sinks his fangs into Jacob's throat, Jacob imprints…he knows that it's the wrong choice, but what can he do about it? He has no say in the matter, and the last thing that he thinks before he's screaming in pain is Why him?
He tries to hold back his screams, they make him feel weak, but he can't help it. The bite hurts, and Jasper can't stop, because now that he's tasted the blood, he's blocked out the screams and everything else. The pain from the venom is mind numbing as it sears through Jacob's body, trying to change him when there's no hope of that ever happening. Alice cringes; she's heard screams before, but she can't handle this – Jasper, her Jasper causing the screams of an innocent creature. She closes her eyes as Jacob writhes in pain, his eyes clenched shut as he screams.
"That's enough!" The being orders, practically pulling Jasper away from Jacob. The werewolf falls limp, held up only by the chains binding him to the wall. He's still alive, but he's weak, and if he thought his body hurt before, now he was fucking dying. The pain renders the werewolf unconscious, and he becomes unimportant as his blood immediately begins to affect Jasper. "Lock him up!" The being orders the second Jasper begins to fight wildly against his captors. They drag him across the room, throwing both him and Alice into a confined cell.
"Jasper?" Alice looks scared as she backs into a corner. "Jasper, I love you," she whispers. She could beg Jasper to stop, but he wouldn't hear her, because the real Jasper is trapped somewhere, his body ruled, dominated by the forbidden blood. She could try to fight him off, but she's seen the future, and this is her end, so she lets it come. Jasper tears her to pieces, not even conscious as to what he's doing until long after it's done.