A/N: Hello, my friends! This story is mostly Aini NuFire's fault, haha. She's the one who sent the plot bunny my way, as well as being a great beta reader for this, so many thanks for that! This is an AU, set in season 11 right after Beyond the Mat - Crowley has only recently escaped Lucifer, and the boys have found out that their angel has been taken over. It just seemed to me that Lucifer was taking it easier on Cas than I would have imagined, so I re-wrote the rest of the story with a little more whump, and a lot more archangel ;)

Also, I love Chuck, I do, but he won't really be featuring in this particular AU. This fic was born before he showed back up, so he wasn't written into it, haha.

I don't own any canon Supernatural characters. Enjoy!

Chapter 1 – This is the Story of an Angel

The hunt for the Darkness wasn't making any progress at all, and Lucifer was getting annoyed. The weapons brought to him by the demons were more or less worthless, his own strength—magnificent though it was—wouldn't be enough to cut it, and Amara was probably getting stronger every day.

Not to mention that his favorite pet, Crowley, had escaped, and the Winchesters still weren't dead. What he needed was a win. Several thoughts had been turning over in the dark archangel's mind… Dean Winchester's connection to Amara, for one. The realization that he would probably need back up after all, and that his options there were slim, was irritating.

Shadows flickered on the wall of the crypt, flames from the torches dancing off the bronze and gold weapons that had been piled in one corner. A handful of demons lined the stone walls, talking quietly amongst themselves. In his chair, set up on a dais like a throne, Lucifer rubbed his forehead.

"Why has no one brought Crowley back yet?" he demanded out loud, bringing a nervous hush from the demons in the crypt. One was nudged forward by the others as the elected spokesperson; it gulped, looked around for help, then stammered,

"My- my lord, we're doing… everything we can to find him."

"Really?" Lucifer asked, raising an eyebrow. "Looks to me like you're all just standing there." He got to his feet, and the demons shrank back in fear. Lucifer smiled, prowling closer. "So… new plan. You lot, get out there. And when you come back, you'd better have either Crowley, another heavenly weapon, or the Winchesters. I need something to kill. So I'd suggest you not come back empty handed or I'll have to improvise."

The demons traded looks, baffled and fearful in the muted light, but didn't seem eager to make a move. Lucifer rolled his eyes, then pointed to the exit.

"Now!"

They scrambled for the door, pushing and shoving in their haste to escape his impatience. Soon, the Devil was alone in the crypt. He moved back towards his chair, a deep glower etched into the borrowed face he wore. And speaking of this borrowed body…

"Castiel?" he drawled, the glare smoothing into a smug expression. "Something you want to say? I feel you squirming around. Here… we're alone, I'll take the gag off for a minute."

The Devil shifted inwards, into Castiel's mind where both of their souls existed. The outer world faded slightly, a luminescent, shimmering image cast onto the mirrors that surrounded the small prison he'd created in the lesser angel's mind. Castiel was kneeling in the center, inside a sigil Lucifer had drawn on the floor of the mind-realm. A heavy yoke laid across his shoulders, manacles attached to either end that locked Castiel's hands outspread, while a silver strip of cloth was bound over his mouth to keep him quiet. The lesser angel flinched but didn't shrink back as Lucifer reached towards him and pulled the gag down.

"Don't," Castiel rasped as soon as he was able. His expression darkened as he glared up at Lucifer. "Don't hurt the Winchesters, or…"

"…Or?" Lucifer repeated, waiting with an expectant grin. He raised his eyebrows. "You'll do… what, exactly?"

The lesser angel didn't seem to have an answer, though there was still a spark of defiance in his gaze that amused and irritated the Devil. He shook his head, turning his back on the imprisoned angel to look out of the mirrors into the crypt where their physical body sat.

"You know, I still don't get you," he said, hands tucked into his pockets. "I mean, I've seen your memories with them. They clearly don't care that much about you. So why this blind devotion?"

"They're my friends," Castiel ground out from behind him, though his voice was labored and his breaths heavy. "I can do… this much."

"Yeah, no, I've seen your great plan," Lucifer reminded him, turning back around with a dark smile as he tapped the side of his head. "Don't forget, I'm up here. I've heard your thoughts. You figured, what, you and I would save the world, then you'd hold me off long enough for Sam and Dean to take me out, right? That was the idea?"

The lesser angel paled, and Lucifer savored the panic in Castiel's eyes. He chuckled. "You thought I didn't know? You're right about one thing, though. They would try. Now, if it was dear Sammy they'd have to sacrifice…" He tilted a hand back and forth, wincing to show doubt. "That'd be a tough one. But since it's just you… yeah. They wouldn't hesitate. Wouldn't even blink."

He grinned when Castiel looked away, jaw clenched. It was that exact sort of feeling that had made the lesser angel willing to be a vessel at all, so hey… Lucifer would play it up.

"Here's the thing," he said, strolling casually around the bound angel. "They fear me way more than they care about you. So you're pretty much screwed. Anyway, doesn't matter. As soon as I've dealt with the Darkness, they're next on the list. And believe me, I'm going to enjoy stripping the flesh off their bones and feeding it to them, one piece at a time."

"No. I won't let you," Castiel snarled, making Lucifer laugh louder.

"I- I'm sorry," he said, wiping at his eyes. "But…" The Devil paused to chuckle again. "Sometimes, you- you just say the cutest things. No, you don't have a choice. Interfering is what landed you in this position to begin with." He gestured to the yoke and chains, shrugging. He could have killed the damn Winchesters already if Castiel hadn't temporarily stolen control back, but Lucifer had ensured that wouldn't be happening a second time.

"I'll revoke my permission," the lesser angel warned, shifting against the weight of the yoke. Lucifer snorted.

"Good luck with that," he drawled. "I mean, you're pretty much useless. You're a nice little reservoir of grace to keep our body healed, but you're not nearly strong enough to kick me out… and you know it."

Castiel exhaled a ragged breath, sweat beading at his temples. He caught Lucifer's gaze again, but didn't argue. The archangel could hear Castiel's thought of agreement, though, which was satisfying enough to make up for the loss of his pet demon. Lucifer smiled again, looking around the mind-prison with an approving nod.

"Gotta say, it never occurred to me to use an angel as a vessel, but I kinda like this arrangement. I mean, we don't even have to drink demon blood to keep our body strong… as long as you keep healing it."

After all, it took a lot of power to keep the vessel they were in from exploding under the pressure of bearing two angels, one of which was himself. Healing was normally automatic for an angel… but this wasn't a normal situation. It took effort, but fortunately he had a captive copilot.

"You could… help," Castiel complained. His hands clenched and unclenched in their chains, but they and the sigil he was in kept him from moving around much. "I- I can't… it's draining me."

"That's the idea," Lucifer pointed out, rolling his eyes. "You just keep healing us, Castiel. Keeps you at low power, less able to cause any more trouble. Besides, makes sense for me to keep my grace at full, so I can wipe out the Darkness. I mean, that was the whole point of you saying yes, right?"

"Then why haven't you made a move? I can't keep this up forever."

"Oh, I know," the archangel agreed. "No need to worry. When you're just about tapped out, I'll give you a little refill so you can start all over. We could pretty much do this indefinitely." Which was, of course, the plan. Though, the permanent state of exhaustion would probably destroy Castiel's mind after enough millennia, but that was what happened to angels who sided against Lucifer.

The lesser angel slumped down again, his entire mind reeking deliciously of despair, which made Lucifer laugh and reach out. Castiel didn't meet his eyes as the Devil cupped his chin, pulling his face upwards.

"Poor little angel," Lucifer crooned. "Can't even sacrifice yourself properly, huh?"

Castiel's throat bobbed, a fine tremor wracking his body under Lucifer's fingertips. It gratified the archangel that Castiel was this easy to torment, relieving some of his own frustration. Releasing his grip on his brother, Lucifer stepped back and looked around at the mirrored walls. He lazily shifted the scene displayed on them one panel at a time.

"You know," he said, as the view of the crypt was replaced by the interior of the bunker bit by bit, "I keep seeing this in your mind. What's got you so fixated on this place?"

Castiel shifted, a spark of anger lighting his face. "Stay out of my head."

"Says the one who literally invited me in," Lucifer shot back with a snicker. With a wave of his hand, the archangel brought up ghostly images of the tables and bookshelves, which solidified around them until the two angels seemed to actually be in the Men of Letters bunker.

When Lucifer glanced at Castiel again, the angel couldn't have looked more downtrodden, hands subtly straining to free themselves of the manacles. Lucifer let him stew for a second, before exclaiming with delight,

"That's right, this was supposed to be your… what's the word? Safe house! You thought this would be a home for you at last… right? I mean, you had nowhere else to go. So, uh…" He cocked his head, grinning. "How did that work out? Did you get your home sweet home? I'm guessing not, since you ended up here."

"Stop it."

Seriously, it was too easy. Lucifer grinned, strolling along the bookshelves, eyeing the titles from Castiel's memory. One lazy finger dragged along the spines as he went, letting the books smolder and then catch fire, until the entire line of bookshelves was burning.

"Look at that," Lucifer said, eyeing his handiwork as he came back to stand beside Castiel, giving the angel a hard nudge with his foot. "It's symbolic, right? All your dreams going up in smoke."

He grinned at the crackling sound of dusty old pages burning to a crisp, mixed with the increased distress in Castiel's labored breaths. Lucifer let the vision continue, the fire spreading to the floor and climbing up to the ceiling. Plaster and ash rained down on them as the bunker began to collapse, erasing whatever place of "safety" the lesser angel had thought he was getting.

"Stop," Castiel growled again, almost tearful but not quite. "Just… just stop."

Lucifer shrugged, then waved his hand again. The fire disappeared, revealing the bunker unscathed. "You're right," he decided. "There's a lot of knowledge stored in here. Seems a shame to let it all go to waste. Maybe after I kill the Winchesters, I'll take their home. You'd get to live in it after all, Castiel! Just… with me. See? I can be nice."

He smirked down at the lesser angel, who didn't appear to share this sentiment. Lucifer stuffed his hands into his pockets, looking around the room. "Maybe I won't even kill them," he decided. "With Crowley in the wind, I've lost my dog. If you groveled really nicely, maybe I could be persuaded to give them his spot. What do you think?"

Castiel didn't answer, but turned away with a glower. The yoke on his shoulders creaked as he pulled at the restraints, but he wasn't going anywhere. The lack of response irritated Lucifer, though. He slammed a hand onto the heavy yoke, making the lesser angel jump with a soft gasp.

"That was a question," Lucifer pointed out, voice deadly calm as he twisted the wooden bar so that Castiel was forced to tip his head up and look at him. "Would you rather I keep them alive? I'd do it, I swear. Ask me to make the Winchesters my new puppies and leave them crawling around on all fours like Crowley, licking up their food from a bowl. I'd do that for you, Castiel, because I care about your happiness."

Still, the angel said nothing, just glared at him with jaw visibly clenched. Lucifer's smile spread, icy and dangerous. Nothing would be so satisfying as to hear Castiel actually plead for him to keep those precious pets of his on a leash, especially because Lucifer was certain the humans would prefer death to that kind of degradation.

"Nothing?" he asked with a mock pout. "Come on, Cas… a little groveling goes a long way. You could save them."

"When this is over," the lesser angel finally answered, steady and cool. "The Winchesters will kill us both. I believe in them."

Lucifer sighed. "You're no fun."

"Crowley was right, wasn't he?"

The sudden change in topic made Lucifer frown, crossing his arms as he regarded the captive angel. "Meaning?"

Teeth gritted, Castiel ground out, "I only said yes because you told me you could defeat the Darkness. I thought we were going to fight her, to bring this to an end. All you've done is sit around and gather weapons that won't work and make vague threats about 'after'. Crowley was right… you're not strong enough, are you?"

Lucifer's face tightened, Castiel's captured angel blade slipping into his hand. Again, the lesser angel flinched as the tip of the blade pressed against his cheek.

"You know," Lucifer suggested, sliding the blade to Castiel's mouth. "Instead of gagging you, I could always just cut out your tongue." He twitched as though to make the cut, smirking when Castiel inhaled sharply and closed his eyes. "I will defeat the Darkness. I have a plan. Something I was hoping could be avoided, but… it seems I could use some backup."

And on that subject, Castiel was losing his entertainment value. Time to get what he'd come in for in the first place.

The blade disappeared as Lucifer pressed his hand against the lesser angel's forehead instead. Castiel grunted and tried to squirm away.

"What are you doing?" he asked in alarm. "Lucifer… wait!"

"Like I said," the archangel snapped, gripping the yoke with his free hand to hold Castiel still. "I have a plan. But it's going to take a lot of power, so… I'll be siphoning yours."

"No, I can't heal us if I lose much more!"

Light flooded the room, beaming out from beneath Lucifer's hand as heavenly energy slid from Castiel into the archangel. He could feel his brother weakening, but didn't stop until he'd drained enough to bolster his own stores considerably, enough to accomplish what would be no easy feat. Pure energy filled his veins, and Lucifer laughed, high with the sheer feeling it brought.

"Oh, Castiel," he exclaimed, pulling his hand away and chuckling again as the lesser angel slumped and nearly toppled. "I'm so glad you agreed to this."

"L-Lucifer…" Castiel whispered, eyelids fluttering as sweat dripped down his pallid face. "Please, I'm… almost out…"

"You'll be fine. As soon as I've done what I need to, I'll give you a little top-up as a reward, okay?" Lucifer grinned and patted the top of Castiel's head, pleased and energized. He paused, though, catching sight of the back of his hand with a frown. Spots. The skin was starting to peel. Raising an eyebrow, he pointed out, "Uh, that doesn't mean you get to stop healing us. No slacking, because if we die now, then the Darkness still wins and kills your humans, and then all of this was for nothing. Shake a leg, Castiel, I left enough for you to keep working."

"I… I'm trying…"

"Try harder," Lucifer suggested with a shrug. "I'll be back soon. Meantime, you don't need the distraction of everything going on out there."

A snap of his fingers, and the image of the bunker disappeared. Blackness blanketed the room in impenetrable pitch until the only source of light was Lucifer himself, radiant with the splendor he'd had of old. The archangel smiled, turning to Castiel.

"I've muted the sound, too," he explained. "So don't bother trying to eavesdrop. Just do your job while you still have any value to me."

"Why… why are you doing this?" the depleted angel whispered, head still bowed as he fought to hold their body together.

Lucifer leaned down, shoving the silver cloth between Castiel's teeth so there'd be no concern of him speaking through their vessel's mouth. Then, he gripped the angel's chin once again, forcing his drooping head up.

"Because," he answered with a dangerous smile. "I'm the Devil. And you knew that when you let me in."

With his retreat, the light also retreated. Lucifer pressed his consciousness back out of Castiel's mind, into the waking world, leaving the captive angel alone in absolute darkness and silence with his feebly flickering grace.