Disclaimer—Supernatural isn't mine. If that comes as a surprise to anyone, I have some lovely swampland in Florida that you should take a look at.
Sam's shout was still ringing in the air when the demon pulled an elaborately decorated knife out of its robes and turned towards Castiel. "There will be a sacrifice, and my master will rise!"
"Like hell!" Sam snarled, turning his back on his brother's limp form. A flare of fire from the object in his hands knocked the demon sideways a few steps, but if holy water and an exorcism hadn't been enough to drive it out, it was far too powerful for anything physical to affect.
The demon shook itself and gave Sam a disgusted look. "I'm going to kill the angel first, but don't worry, you and the rest of these meatsuits will have your turn." It started to continue its advance towards the altar but unexpectedly grunted and faltered.
Even with his senses damped to nearly nothing by the bindings, Castiel could feel the sudden taint of evil on the human boy as he stood with his arm outstretched. Sam's power had been enough to stop the demon, though. Partially, anyway; the thing turned towards him and away from Castiel.
"Well, well, well." The demon took on a mocking tone. "One of Azazel's little monsters. The last survivor, if I'm not mistaken." Its lips curled in disgust. "Unnatural. A mockery. None of you should have been allowed to live."
Sam's arm wavered slightly. "You know what I am?"
"Of course. You were a big part of his plan. Of course, he's gone now, but you aren't. Unfortunately."
"What is this plan?" Sam demanded. "Why did he do this to me? Why did he do this to any of us?"
Something that Castiel wanted desperately to know as well, but the demon just chuckled. "Oh, you'll see. Soon enough." It shifted its grip on the knife and started to turn. "But that's not my concern at the moment. You shouldn't fight me, little monster, you won't win."
Sam's eyes flicked towards Castiel for a brief second, and then his jaw clenched and his arm steadied again. "Watch me."
There was no fierce battle, no dramatic gestures aside from the Winchester boy's outstretched hand, just the locking of eyes between the two figures and the feeling of evil in the air. At first it seemed fairly evenly matched—neither so much as twitched a muscle—but then blood began to pour from the Winchester boy's nose as his eyes went dark, and the demon's body jerked. And then it coughed and emitted a puff of black smoke. Followed by another, and another. Sam Winchester was obviously in pain, as well, though; when the body the demon had inhabited finally sank to the floor, Sam doubled over almost immediately, hands on his knees and his head hanging down.
For a long moment, Castiel thought Sam was going to collapse beside the man that the demon had inhabited, but somehow he managed to force himself upright and stagger towards his brother.
Castiel couldn't deny that he felt relief himself when Sam knelt beside Dean, touching his brother's neck, and obvious relief crossed his face.
After a few minutes of light prodding to Dean's neck and back, Sam heaved himself upright again. He had Dean balanced rather precariously over his shoulder, and it wasn't precisely a graceful procession as they made their way towards the door. They did make it out, though, which was more than Castiel was managing.
The victims that had been possessed—those that had survived the possession, anyway—were starting to rise as well, making their way out of the room in a confused mass. None of them spared Castiel so much as a glance as he resumed his struggles.
Despite his best efforts, he was no more successful now than he had been before. The chains wouldn't release, he could feel no loosening in the manacles, and it seemed that every twitch of his muscles was rendering him weaker and weaker.
"Here, let me try."
Castiel jerked back as best he could as Sam Winchester scrambled onto the altar in front of him, a metal bar in one hand and an odd-looking knife in the other. The immediate taint of evil had faded, but his face and the front of his shirt were still covered in blood and Castiel couldn't forget what he had just seen. Even the demon had identified the boy as unnatural.
Sam shook his head before Castiel could say anything. "Look, I get you don't like me much, all right? And you probably like me even less now than you did a few minutes ago. But Dean's unconscious and they're…," he waved a hand as the last of the survivors finally made it out the door. "Well, they're not going to be a whole lot of use to anyone for awhile. So unless you've got a way to get yourself down that for you just haven't chosen to use, your options are letting me help or hanging out here." He frowned. "Literally. And there will be demons coming back, probably sooner that later—at least two of the ones acting as guards outside managed to escape." He turned and looked down at the doorway drawn upon the floor. "No offense, but I really don't think your odds are good if you stay here."
A rather unassailable point. Castiel gave as much of a nod as he could manage with one of the chains holding his neck to the wall. "Agreed." Although he didn't particularly like it.
"Thank you." Sam tucked the knife into his belt and jammed the bar into a link in the chain holding Castiel's right wrist to the wall. Gritting his teeth, he pulled it downwards, and the chain gave with a groan.
Sam reached for the manacle itself, and Castiel barely refrained from pulling his hand away. "I don't believe you will be able to remove it with that bar."
"I doubt it. This looks almost fused to your skin. "Is this why you can't get down yourself?"
"Yes. The manacles prevent me from accessing my powers…at the moment I am limited to purely mortal means." The demon presiding over the sacrifice had attached them, imbuing them with the power and Marks to keep him bound, and he wasn't entirely certain that there existed a mortal object that could release him from them. He was just fortunate that the demon had put no such protections on the chains.
Sam stared at him for a moment and then leaned closer to examine the band before releasing Castiel's arm with a shake of his head. "If it's all right with you, I'd like to worry about getting out of here first, and then we can deal with those later."
Sam moved on to the next ring in the wall, the one attached to the manacle around his neck. "Hold…really still."
As though Castiel had any other choice. The bar slipped once, scratching his neck, but he dismissed Sam's immediate apology with a minute shake of his head. It was hardly his most pressing concern.
He sagged forward abruptly, forehead landing on Sam's shoulder, as the ring holding his neck to the wall released suddenly and he discovered that the only things still keeping him upright were the chain on his left wrist and the minuscule strength left in his legs. The fight earlier, as well as his struggles to free himself, had left him weak and drained, and while normally he could draw upon his powers to replenish himself, that wasn't an option at the moment.
"Easy," Sam said, pushing him back against the wall. "Just one more to go."
That 'one more' took more time than the first two, mostly because Sam had to support him as well as break the chain, but eventually the metal gave and Sam climbed back down off the altar dragging Castiel with him.
It took far more effort than Castiel cared to admit to keep his legs under him, and if Sam hadn't been more than half carrying him, he would never have made it out of the building.
He saw Dean lying, still unconscious, in the back seat of the car as Sam helped him into the front seat.
"Here." Sam gave Castiel the knife that he'd tucked in his belt earlier and set the bar down by his feet before grabbing a bag out of the back seat.
Castiel examined the blade for a moment. "What is this?" What good would a knife do him now?
"It kills demons. Just in case. I'll be right back."
"Where—?" Sam had already turned back for the building before he managed even that single word. Castiel had no idea what it was that he planned to do, but only a few minutes passed before Sam returned, climbing into the driver's seat and tossing the bag onto the floor of the back seat. He took the car out of the lot with a squeal of tires, and in the reverse-facing mirror, Castiel could see a plume of fire rising from the warehouse.
"You burned the doorway?"
"Yeah, and stole their knife." He produced the blade from his jacket and stuck it in a compartment in front of Castiel. "Just to be safe. If they got you, they might be able to get their hands on one of your friends too."
A valid point. He cocked his head. "Would you have left me there? If I had refused your assistance?"