demon!Dean, angel!Cas, AU up the wazoo, rated M for language, future violence and gore, and possible smut in later chapters. Or not-so-later chapters, depends on how easily tempted I'll be. I've never written smut before.
btw, mentorships are like scholarships for angels. One senior angel offers to be their mentor, to train them in the ways of demons and fighting and whatnot. Usually happens at college age, after primary education. AU thing.

"Open the skies over me,

I am waiting patiently,

I'll wait for a sign,

As conspiracies unwind,

Will you slam shut, or free your mind,

Or stay hypnotized?"

- Inspired by Exo Politics by Muse -

As it turned out, Castiel was not a brilliant example of an angel, as today's lesson had proved. The task he'd been sent was easy enough - he had to go down into the upper levels of Heaven's prison cells, scathingly nicknamed the Underworld, and speak to a demon, the better to learn of their nature.

Not that there was much to learn. According to his teacher Uriel, demons were all as unique and individual as matches in a box. All were vicious, vindictive, compulsive liars. You couldn't trust them further than you could throw them, and in Castiel's case that wasn't very far at all - he wasn't yet in the full bloom of his angelic power, unlike Uriel, who could probably smite him and all of his fellow students at once if he wanted to, or possibly just beat them to death with his massive wings. He was immensely powerful, and Castiel was proud to be one of his students.
But not a worthy student, apparently. He'd found his way to the Underworld with ease, aided by directions from a few angels he passed, but now he was faced with the task of finding his demon, and it was proving more than difficult.
After a while, every door in the hallway started to look the same, except for the four digits printed in crisp black lettering above the handle. Within forty minutes of entering, Castiel was hopelessly and embarassingly lost, and he'd forgotten the number Uriel had given him. Why the hell hadn't he written it down, and what the fuck was he supposed to do now? His heart started to thump erratically as the lookalike doors streamed past him. 0648, 0649, 0650...
There were stories about the Underworld, the sort that were usually told around a campfire in the woods. These tunnels were so labyrinthine that people had gotten lost in them and starved, spending the short remainder of their lives huddled and freezing against the wall, taunted and tormented to their deaths by demons. Castiel didn't believe the freezing-to-death part, though he could admit it was pretty chilly in there; his wings were shivering. He could starve, though. He was going to starve. He was going to die, cold and alone, and worse, he was going to die a failure, and-

Shut the fuck up, assbutt. You're a delusional pessimist and you know it. Uriel will come looking for you. Your family will come looking for you. Hell, it'll be the first time an angel vanishes from Heaven in how long now? Every angel in the goddamn world will come looking for you. And Uriel knows where you are anyway.

He was being a pessimist, it was true. Uriel wouldn't even look at him if he knew Castiel was doing this to himself - if he heard about it, he would simply write a suicide note of shame before taking his own life. This wasn't how an angel should think. He wasn't a toddler lost on the way to the toilet. He wasn't a child. Not anymore.

First thing's first. Calm yourself, then try to remember your cell number. You're not thinking clearly. When you're done, you can just follow the doors back to zero, can't you? Dumbass.

He breathed in deeply, then exhaled slowly, feeling his heart settle back into a more natural rhythm. He really was being stupid.
Starting off down the corridor with considerably more courage, he scanned the numbered doors, waiting for the sense of familiarity that was bound to rush over him when he saw the one that was for him.

0651, 0652, 0653...

Nope. Not even remotely familiar. He knew it was definitely in the six-hundreds...

0654, 0655, 0656...

Ah. That last one brought a sense of deja vu, but when he slid back the cover over the peep-hole at the top of the door, he saw the room was unoccupied.

Ten more doors, then.

He skipped ahead for a bit until he came to cell number 0666.

That's not even slightly ominous.

Feeling like his heart was in his throat, Castiel laid a hand on the door-handle. A demon was inside there, waiting for him. A real, live demon. With his luck, it would be one of the savage ones that were barely capable of speech, and even if it wasn't it would probably just scream obscenities at him until his hour-long quota was finished. This would be the first time Castiel would see a demon in person, instead of in a training video or through a memory shared by another angel. And he was scared shitless.

Fishing the key Uriel had given him out of his pocket, he inserted it into the lock and turned it, hearing the click as the door unlocked before noticing the tiny number 0666 engraved on the keyhandle. Huh.

He breathed deeply again, bracing himself, before he swung open the door and closed it behind him much harder than he intended, so that it slammed with a loud thud that almost made him squeak. He stuffed the key back in his pocket self-consciously.
The room was not the terrifying torture dungeon he had expected it to be. The walls were clean and white, and the floor was covered in polished chrome tiles. Everything was spotless and unmarked, except for a small three-metre circular design - a devil's trap -engraved in the middle of the floor, at the center of which was a black, padded leather chair, on which reclined the demon, looking as if he didn't have a care in the world.
His first thought was that there had been some kind of mix-up, a mistake, because the thing sitting in front of him could not be a demon. He'd seen them in training videos and in memories shown to him by others. They had cold, lifeless eyes, not green ones, and they shouted and raged at their captors, and they certainly didn't smile at anything except the pain of others - not like this man was smiling at him now, as if he was genuinely pleased to see the angel, like he'd been waiting all day. Castiel had to admit, it was a dazzling smile, too. It made him feel instantly at home, so much that he had to force himself to be unsettled again, rather than play into the demon's hands.

"Oh, hi." The demon drawled, staring lazily up at the ceiling. "You're the first visitor I've had in ages. Came for a chat, huh? Took you long enough to get here."

That ruffled Castiel's feathers, and that little bit of irritation was enough to shock him out of his stupor.

"I'm sorry." He said innocently. "Is there something else you need to be doing right now?"

The demon's eyes darkened as his eager grin dissolved, and he said nothing, merely glaring at a point a few inches in front of Castiel's feet until the angel seemed sufficiently intimidated.

"Now are you going to talk, or shall I tell you something about myself?" The demon mocked him, without looking up.

"Your name first, please."

The demon chuckled. "Manners. Haven't seen a cent's worth of those since I came here. We demons second-class citizen in Heaven, 'case you hadn't noticed." He paused, his eyes darting up to meet Castiel's for the first time, and the angel felt something like an electric shock zip through him, as something in his stomach fluttered uncertainly. Those eyes were really fucking green-

"My name is Dean Winchester." He said, bringing Castiel out of his little fangirl moment. God, what was wrong with him?

"I-I'm Castiel." He stuttered, feeling a flush start to creep into his cheeks.

"Pretentious, no surname, and ends in -el. Yeah, that sounds right for an angel."

He was blushing, oh fuck, he was blushing. He must be sick or something. This was a demon. How could Dean not be seeing him like this?

That might possibly be because Dean wasn't looking at him. He stared at the ceiling again with something akin to deep-seated boredom.

"It must be dull in here." Castiel commented, desperate to break the silence. He was losing control of this situation - it was like Dean was the jailor, not the prisoner.

"You have no idea." Dean replied, that easy grin flashing across his face again. "I wish they'd put up some pictures or something. A nice beach mosaic..." He trailed off. Castiel didn't speak for a while, processing what he had already learned from speaking to Dean. He noticed that Dean's gaze had returned to him again, and this time it wasn't moving - actually, the intensity with which he was staring made Castiel's skin prickle, but it didn't feel like fear or disgust. Dean didn't seem abhorrent at all. In fact, Castiel personally thought he was- but no. He was going to stop that thought right there. Demon, Castiel. He's a demon.

"I expected you to be more..." He struggled to find the right word.

"Demonic?"

"... Yes. I suppose so."

"Racist." Dean chortled, before continuing. "Do you want me to do the eye thing?"

"If you think it'll help." What the hell.

Dean smiled at him, and this one was more predatory than friendly, exactly how Castiel would have expected a demon to smile before entering this room. Then, gradually, Dean's green eyes started to flood with darkness from corner to corner, until the whites and irises disappeared entirely, and nothing was left but shiny, liquid black.

Castiel couldn't help the tiny, barely audible gasp that worked its way out of his throat. Dean's eyes - they weren't cold or lifeless. They weren't like icy water at all, as Uriel would have described them. They were more like dusty, smoking charcoal - the heat and ashen warmth behind them seemed to spark something slow-burning in Castiel, too. He couldn't look away from them, and the more he looked, the warmer he became. He couldn't read Dean's expression as easily anymore - the change had robbed him of his most expressive feature, but Dean wasn't looking away, either.

Their stares held for what seemed like minutes, then Castiel's eyes dropped swiftly to the floor as he felt his face start to burn. What was happening to him? Why was he getting all poetic about a demon's eyes?
Yes, remember, Castiel. He's a demon. You don't think like this about demons. You don't think like this about guys, for that matter.

"Are you scared yet?" Dean asked in a low voice that made Castiel's heart trip.

"No."

"You will be." Dean stated, a slow, vicious smile spreading over his face. He didn't bother returning his eyes to normal.

No, no, this is not going to work on you, nope.

"What are we supposed to talk about in here, anyway?" Castiel said, as casually as he possibly could.

"No idea. What did you learn at school today? What's your favourite colour? What the hell is with that trench coat?"

"I don't go to school, Dean. I'm seventeen. I have a mentorship."

"Really? 'Cause you look about six in that thing." Dean rolled his eyes when Castiel scowled at him. "Right, right, sorry. Answer my other questions, please?"

"My favourite colour is blue. And the coat belonged to my father."

Dean gazed at him for a moment, looking like he wanted to enquire further, but he didn't, and Castiel was grateful for it. That subject wasn't something he wanted to talk about, particularly with a demon.

"Why blue?" Dean asked softly.

"Because it's the colour of my mother's eyes, and mine, and most of the things in our house. My mother's favourite colour is blue." Castiel explained, a small smile gracing his face as he watched Dean's face flicker with unusual interest. He wasn't sure why he was telling Dean this - perhaps because it didn't matter. In all likelihood, Dean would never tell anyone about this, and Castiel would probably never see him again. The thought made him a little sad, for some reason.

"She sounds nice."

"She is. Her name's Isis."

To Castiel's dismay, another awkward silence ensued. How in hell did they start talking about his mother? Dean didn't seem to notice, though - his eyes stayed fixed on Castiel's face as though he was trying to memorize the sight.

"Do you have to go soon?" He asked suddenly, and Castiel's stomach did a little flip.

"I only have an hour." Castiel said hurriedly. "But I can make up an excuse to stay longer... If you want?"

What the fuck was he doing?

Dean positively beamed at him. "I like the way you think, angel."

Fuck, that should not make him as happy as it did. This situation was quickly spiralling out of control.

Dean kept asking him questions long after their hour was up. He kindly asked after Castiel's health, pestered him about his wings, begged to try on his coat (Castiel politely turned him down), then promptly enquired as to whether he'd ever done anything kinky with that tie he was wearing (which made him blush furiously, again), then asked to hear more about his mother, Isis, and then the rest of his family and his friends - his sister Annael and his brother Gabriel, his uncle Zacariah, his best friend, Balthazar, and then a host of others he remembered from school, whose names he barely recalled - Nenamiah, Taren, Luporiel, Anifer, Rosariel - fuck, the list went on forever, since Castiel had a good memory and Dean seemed to know when he was leaving things out. He confessed to a crush he'd had on Rosariel when he was fourteen, which he'd never told anyone about, and made Dean grin at him cheekily.

The answers seemed to flow from his mouth whether he wanted them to or not, but not as fast as Dean fired questions at him.
What was his favourite food, what flavour of ice cream did he like most, did he think 'bloody' counted as a swear word or not, what kind of fruit did he like, was his mother was a gardener, did he even have a garden, what kind of house did he live in - the questions just kept coming, in an endless, assymetrical pattern that didn't seem to have any rhyme or reason. And Dean was breathing in the information like every word was edible and he was starving to death; Castiel supposed this was true, in a way - Dean had been cut off from the rest of the world for so long. They talked and laughed together - Dean seemed to produce a neverending stream of alternating good-humoured jokes and innuendos that made Castiel blush every time. By the time Dean finally seemed satisfied that he knew everything there was to know about Castiel, the angel's throat was sore from speaking so much.

At some point he'd stopped standing so woodenly in favour of leaning against the wall before sinking down to sit against it. Dean had flopped out of his chair and onto the floor, lying with his arms folded behind his head. If Dean wasn't confined by the devil's trap, Castiel was convinced the demon would have tried to sit in his lap by now, if the way he was flirting was anything to go by. As it was, Dean was lying as close to him as he possibly could, right against the rim of the circle.

How long had it been? Castiel's internal clock was terrible, so he was beginning to worry.

"I should probably go." He said finally, feeling his insides deflate oddly as he said the words. Rising to his feet reluctantly, he started to walk to the door.

Dean nodded robotically, his dark eyes focused on the ceiling.

"I know."

By that point, the angel had decided. Dean had been alone for a long time, that much was clear, and seeing him like this made Castiel realize it. He wasn't going to leave Dean to rot for another decade, until another, less friendly trainee angel came to relieve his loneliness to some small degree.

"I'll come back. Even if I have to steal the key." Castiel added uncertainly, reaching for the handle. Apparently Dean didn't hear him, so Castiel turned it, and was about to leave when suddenly-

"Goodbye, Cas."

Cas smiled.

"I'll see you later, Dean."

Uncle Zacariah, pfft. That is all.
What? I just thought he looked avuncular, you know? Though he really wasn't. But he was. In a creepy kind of way, though. Yeah.
Meep.

I've never written fanfiction before, so bear with me.

Also, I'd like some feedback if anyone's interested. Is it too long? Too short? Was there anything you didn't like? Review me, please.