The Double

Help.

Sebastian tackled Jackson, bringing him to the ground. He pinned him to the ground and shook him. Jackson punched Sebastian right in the nose, hearing a satisfying crunch. Black blood dripped on to Jackson's face.
"You may be stronger than before, but I know power stronger than anything you might have dreamed of. Join me now, and I won't need to do anything I might regret. Join me now, help me, and you can become one of the most powerful beings on this planet."
"I would never join you and be corrupted. NEVER!" Jackson spit on Sebastian.
"Fine then." Sebastian produced a long knife and took Jackson's whip. Afraid of what was going to come next, Jackson closed his eyes. Instead, he was picked up and slung across Sebastian's shoulder like a ragdoll. He was carried into view of Clary and Jace, and they were about to rush to him when they saw Sebastian. Why hadn't they gone in yet? Jackson wondered. They must actually care for me. Wow.
Sebastian took Jackson off of his shoulder and moved him so that he was in front of Sebastian, and his long knife was just barely touching his neck. "Well well well, look what we have here? My little brother, my little sister, and her golden fiance. How ironic that we meet here again. Don't even think about alerting anyone in there," he said, pointing to the Institute with his free hand. "or else he gets it." Sebastian edged the knife against Jackson's throat, causing a thin line of red to appear. "Just come with me now and you can live. Or else he dies."
"Don't do it guys! Run! Get awa-"
"Shut up!" Sebastian shouted in his ear, taking a piece of cloth and tying it around Jackson's mouth. He drove the knife farther into Jackson's throat, causing him to bleed even more. "Now make your decision. He dies, or he lives and you two come wi-" Jackson kicked Sebastian in the stomach, making him drop the knife, and he ran. Faster than he ever thought he could before. He was going to make it to Clary and Jace! Closer, faster! Both of their seraph blades lit up to protect him.

And his own whip, wielded by Sebastian, coiled around his leg and brought him straight into the ground. His leg snapped, and the pain felt like white-hot nails were being driven through his leg. Sebastian yanked him up by his hair and grabbed his hand with a grip of iron.
"NO!" Clary shrieked, both of them running to get him, but Sebastian had him. He twisted his ring and they disappeared.
When Jackson woke again, he was chained to a bed, with Sebastian looking over him.