Disclaimer: I do not own Supernatural

This has been sitting in my laptop for a few months now. It was orignally for a prompt on livejournal and I just found it again and fixed it up a little. So, this was my attempt for it and, as I thought about it more, I think there's a bit more I can still add to this at a later time. We'll see how things go but for now it's complete. I'll post the full prompt on the bottom. It's a bit dark, and I warn you there is some torture and knifeplay in here.

It was like an itch he couldn't quite scratch in the beginning. The monsters, they would all just come at him, full force and he'd take them all down with Cas' assistance. Slashing and cutting. Using a gun while he still had a few bullets left. He was, after all, only human. Fragile. Then, when the days were done (or whatever counted as a day) they would sit in a safe place and Dean would notice Castiel staring at him worriedly and start muttering to himself. Dean knew it wasn't normal but he let it slide on account of Cas' crazy sometimes making a surprise appearance. Day after day, hour after hour, Dean and Cas would work tirelessly on "ganking" some monsters and then try to find a way out of this nocturnal craphole universe.

Until one day.

On one day when a little bitch beast got a little too close and wasn't fast enough in their escape. Dean had her pinned down beneath him when suddenly he felt a smooth, cool blade placed reverently into his hand.

"Go ahead," Castiel whispered hotly into his ear and Dean smirked.

"Looks like it's not your lucky day," Dean said to the creature before he began.

Dean hummed under his breath as he touched the knife against her exposed neck in a curved line down to her navel. She hissed in pain and belatedly Dean realized it was a silver knife. Beautiful. He knew Castiel was still watching him and he wanted to give the angel a good show. The second time he traced the knife against her stomach in a straight line drawing her blood and the bitch screamed. Dean sighed in ecstasy and continued to hum as he meticulously continued to carve.

Dean knew he should've been more upset by his actions but, looking back at the writhing body - no, Dean felt good. Strong. Better.

Castiel watched him from a distance and Dean, covered in blood, turned and gave the angel a smirk of satisfaction.

The first time Dean used telekinesis was a bit of a surprise but he rolled with it. After all, he'd never felt that sort of power before. It was...intoxicating. When his eyes sought out Castiel, the angel frowned while studying his face, made some kind of a decision, and vanished.

"Thanks a lot Cas," he said to empty air.

Something came up behind Dean's back and on instinct he clenched his hand into a fist. He began to hear choking. He could get used to this.

Castiel wasn't gone for long. When he reappeared, Dean was busily sharpening and cleaning one of his two remaining blades.

"Where'd you go?"

Castiel flinched from the screeching noise but then began to speak. "Dean, we need to talk."

"So then talk, angel."

schwick schwick

"When we arrived in Purgatory I was unsure of how the world might affect you. You being human after all."

"Mhmm."

schwick schwick

"I see now that, as time goes by, there is something different. About you."

Dean stood up and idly flipped the blade in his hand. Blade to handle to blade to handle. "And what's so different about me?"

"Your ruthlessness. Your savagery. Your use of telekinesis. Torturing. Mutilating – the list goes on and on. Dean, none of this is you. None of this is the true man you were."

Keyword, were. "So then tell me Cas," Dean said, getting all up into Castiel's personal space for once, "what does all of this mean?"

Castiel walked past Dean and began to speak. "It began years ago, when I first raised you from hell."

"Yeah, and?"

Castiel sighed. "We were almost too late in reaching you and as such, the state of your soul, it was already almost to the point of becoming fully demonic. Irreversibly."

"Go on."

"My brethren and I had a short discussion and decided on suppressing that part of you in order for you to return to earth and act as the Righteous Man we needed. We buried the demonic part of you very deep into your subconscious in the hope you would not turn against the important task we wanted you to do."

Dean definitely didn't miss the use of the word 'we' in that statement.

"Being in purgatory this long has stripped that block within you and..."

"Being so righteous yourself, why didn't you try to stop it?"

"What?"

"Well Cas," Dean said coming very close into the angel's space again, "you could've just stopped demon me from coming out again and you didn't. Why?"

Castiel looked at Dean's lips and swallowed. "Strategy. If I disappeared for a long period of time, physically or mentally, I did not believe that you would be able to defend yourself from attack. When your eyes first went black I thought..."

"I have black eyes?"

"Yes."

"And…are they black right now?"

When Castiel didn't answer fast enough, Dean ripped the sharpened blade from off his belt and held it against the angel's neck.

"Cas?"

"Yes."

Well this was a revelation. Dean knew that he should on some level be upset about it. This was after all one of his worst fears come to life. But... oh, that power. Now he understood why Sam had hidden his addiction to demon blood from him. He was foolish to give it up. Dean let go of Cas and returned to his seat.

"We've still gotta find a way out of here," he said gruffly. He put away the first blade and removed his second, beginning to clean and polish that one as well.

"Ideas?"

The first night after Dean and Cas got out of purgatory and Cas had disappeared to who knows where was the first night Sam noticed something off about his brother. Like at dinner, he'd asked for no salt on his fries which wasn't exactly weird but just a little…off. And when they'd gotten back to his place, Dean had avoided stepping on the rug and asked Sam where he'd put the salt lines. Sam didn't know exactly what had happened in Purgatory but he knew he'd get the story someday, just after Dean readjusted.

It was just a matter of time.

Dean + Purgatory = Demon!Dean, gen

Speaking of demonic Dean, let's say that while he was in Hell Dean started the process of transforming into a demon. Cas halted that process/cleansed that side of him when he pulled Dean out. But all Cas could do was cover it up - part of Dean's soul was still permanently something other than human. Enter Purgatory, a place built to hold monstrous souls. Dean goes native to survive and sort of reverts back to how he acted in Hell, right up to the back eyes (or white eyes :3).
Cas can be present or not. If he is, I'd like him to not be as bothered by this as a human would be. I love the idea of the angels having their own alien morality - plus, he's crazy.
Maybe Sam rescues him at some point and they have to deal with this new (old?) side of Dean.