Sam shook his head and blinked his eyes, trying to clear the smog from both. As his vision focused, he squinted up to see Dean fighting the man that had captured him earlier, Cole. It suddenly dawned on him that Cole hadn't dropped those keys by accident. Sam had been allowed to escape, so that he could be followed.
It wasn't much of a fight. The would-be assassin may not have believed Sam's earlier warnings that he was out of his depth, but he was sure getting the picture now. Sam watched as Dean pinned Cole up against a car, hitting him repeatedly, pressing the first blade up against his throat. Sam almost turned away; he did not want to watch his brother murder this man. Unlike Cole, he had known what he would be walking into, but seeing his brother like this, as one of the creatures which they both despised, made him feel sick to his stomach. Pulling himself together, he pulled out some holy water and the handcuffs, trying to decide if it was worth compromising his mission to try and save this man's life. However, before he could make up his mind, he realized that Dean wasn't actually going to kill the man, he was walking away. Sam felt a small flair of hope, perhaps his brother wasn't completely gone after all.
Seeing his opportunity, Sam grabbed his flask of holy water and flung its contents at Dean who snarled in pain and dropped to one knee. Sam went for his brother's wrist with the handcuffs but before he could put them on, he found himself thrown backwards by a sudden blast of telekinetic force. Oh great, his least favourite demon trick. He struggled to get to his feet, still dazed and unable to use his right arm for balance. Slowly, Sam pulled out the demon killing knife and held it in front of him. He had no intention of using it to deliver a killing blow, but at this point he didn't have a lot of options for defending himself.
Dean sauntered towards him casually, a smile on his lips. "That's funny Sam, I was just about to say the same thing to you."
Sam lunged forwards with the knife, Dean deflected the attack, but didn't press one of his. "He's playing with me" thought Sam, "just like he was with Cole." Sam attacked again, but with only one arm to strike with, he was unable to break through Dean's defence. Desperate, Sam feigned a blow with his left arm, but at the second transferred the blade to his injured right. Sam's shoulder screamed in agony he swung out with his bad arm. Taken by surprise, Dean didn't move fast enough to block. Unfortunately, Sam was unable to put much force behind the blow, and Dean was left only with a shallow cut across his ribs. Just like it had with Cole, being injured finally seemed to provoke Dean to go on the offensive. He grabbed Sam's bad arm and twisted, and Sam was forced to drop the knife as he yelled in pain. Then Dean, obviously much stronger now, physically threw him across the alleyway.
This time, Sam found himself unable to get up as Dean walked towards him. He was still trying when he noticed another figure behind Dean. It was Cole. He had obviously regained consciousness at some point and was now picking up the demon killing knife which Sam had dropped. Sam watched the sunlight glint off the blade as Cole swung it in an overhand strike and plunged it straight into Dean's back.
"No!" yelled Sam in a panic as Dean sank to his knees in front of him. His mind screamed in rejection of what he was seeing. No, this couldn't happen, he wasn't going to watch Dean die in front of him all over again, not after finally finding him. Yet despite his denial, he was still surprised as Dean got slowly to his feet and turned around to face his would-be killer. His eyes completely black, Dean reached up and pulled the knife from between his own shoulder blades. It fell to the pavement with a clatter. Then Dean rushed forward with inhuman speed and the next thing Sam knew Cole's head was rolling away from his body and Dean was continuing to strike at his body with the first blade, just as he had done with Abbadon. Sam knew then that he had no chance of subduing his brother that day by force or otherwise, and also no choice but to get out of there and regroup. With Dean still distracted, Sam staggered to his feet and ran.
When Sam got back to the bunker, Castiel was there waiting for him. The angel took in Sam's battered face and defeated expression and knew without asking what the outcome of his mission had been. Not sure how to comfort the younger Winchester, Castiel did what he had seen Dean do before in times of great emotional distress. He grabbed a couple of beers from the fridge and passed one over to Sam, who took it with a grimace. He didn't open his own of course, he knew that even with his powers failing, he still had enough grace to not be able to taste it properly. So, he just sat there and let Sam tell his story.
"and then he killed him, and I ran" Sam finished. Castiel could see how tired he was. His shoulders were so sloped he was probably only sitting at half his full height and his voice held that air of hopelessness that Castiel had come to associate with and impending apocalypse.
"So, the knife did nothing" Castiel said.
"Not quite nothing. It seemed to hurt him, but he still healed pretty quickly."
"I suppose that shouldn't be too surprising. Cain was the leader of the Knights of Hell and they could not be killed with ordinary weapons. Even angel blades were virtually useless against them."
"Then we have nothing" Sam sighed, but this time Castiel could hear a note of determination in his voice and he knew that the hunter would not be giving up on his brother.
Late that night, or maybe early the next morning, Sam was still sitting at that table. He was long past exhausted, but he refused to sleep until he had come up with a plan to save Dean. Castiel had left a few hours earlier with his friend Hannah. With his grace failing, the two of them had set out on a mission to find some way of helping the dying angel. Sam didn't blame them, he knew that Cas couldn't help right now anyway, and he needed to be thinking about saving his own life, but losing his only ally left him feeling more alone than ever.
Sam sat in the dark, going over every way he knew of to stop a demon. Exorcism wouldn't work because Dean wasn't possessed. Trapping him in a devil's trap somehow was a possibility, but if anyone knew all the old hunting tricks well enough to avoid them, it was Dean. One by one, Sam thought over all of the powerful demons they had faced together over the years, he thought through every scenario, trying to decide if he could apply it to the current problem, and eventually he arrived at a conclusion. There was only one solution, one way he knew of to subdue a powerful demon, immune to most weapons, without necessarily having to kill it. After hours of getting nowhere, the plan came to him now in a rush. He was going to need to find more demons, and not for information this time. No, this time Sam needed something very specific from them: their blood. It was time to revive a long-buried habit.