A/N: For Miyth. This story is actually a combination of two prompts.
1) In 9x6 when Crowley translated a section of the Angel Tablet, he said the spell that cast the angels out was irreversible, but the way he said it made it sound like maybe it could be reversed, but Crowley just believed it was impossible.
2) Exploring the idea of Cas having a soul.

So here's what I came up with. Thank you to 29Pieces for beta reading!

Disclaimer: Supernatural isn't mine. Also, I've drawn a good amount of dialogue from episode 9x11 "First Born" in this chapter before the story goes AU. Other chapters will feature dialogue from other various episodes, but rearranged to fit this AU twist.

Chapter 1: Blame

Castiel finished spreading the jelly on the white bread, and then carefully put the two slices together, crusts lining up symmetrically. It had been a while since he'd had the luxury of eating a peanut butter and jelly sandwich, what with working that case of angel slayings in Wyoming, getting captured and tortured by Malachi's faction, and then helping Dean chase down an angel-possessed Sam.

Things had calmed down, finally. Sam was safe. Dean was too, though on a self-imposed exile. And Castiel had been allowed to stay in the bunker again. He'd also gotten his grace back. Well, not his grace. Theo's essence simmered within him uncomfortably, somewhat like when he'd experienced indigestion as a human. Yes, it gave him access to angelic powers, but it didn't make him whole, didn't even give him his wings back. Still, it made him useful again, and able to once more help the Winchesters clean up the mess he'd caused with the angels falling. Not to mention Castiel could take over the rest of Sam's healing. In stages, anyway.

Sam was currently out, and Castiel was stealing a moment of indulgence. He took a slow bite of the sandwich, prepared to savor the splash of moist fruitiness and creamy peanut butter hitting his taste buds. Yet the moment the bread touched his tongue, Castiel frowned at the coarse texture. Then came the taste of nuttiness and dirt and…a trace of fungal residue. His face scrunched up further as he chewed. This was terrible…

The bunker door opened and closed with a click. "Hey," Sam said as he descended the stairs.

Castiel forced himself to swallow the mash of unpleasant mush in his mouth. It left his tongue feeling like sandpaper. "Tastes like…molecules." Which was very disappointing. He'd been looking forward to this sandwich, and it was practically inedible. To an angel, anyway.

"What?" Sam asked, joining him at the map table. "What are you talking about?"

Castiel looked at the sandwich ruefully. "When I was human, you know, I had to eat constantly. It was kind of annoying." He dropped the sandwich back on the table.

"Yeah, a lot of human things are pretty annoying," Sam agreed.

"But…" Castiel gestured at the spread of ingredients. "I enjoyed the taste of food. Particularly peanut butter with grape jelly, not jam. Jam I found unsettling." He didn't understand how anyone could eat jam. But, he supposed now it would taste no different to him.

Sam set his plastic grocery bag on the table and then sat on the edge. "So, what? Now you can't taste PB&J?" He dipped his finger in a glob of the mixture from Castiel's plate and put it in his mouth.

Castiel watched him enviously. "No, I-I taste every molecule." He dipped his finger in the peanut butter and tentatively touched it to his tongue, hoping maybe a whole bite was just too much at one time.

He grimaced. Nope.

"Not the sum of its parts, huh?" Sam asked.

Castiel shook his head. "It's overwhelming. It's disgusting." He sighed. In the grand scheme of things, it was a small price to pay for becoming an angel again, but still… "I miss you, PB&J."

Castiel pushed his chair back and stood up. "We need to continue your healing. We're almost done."

Sam was doing very well, actually, and didn't even show any outward signs of illness anymore, but Castiel would still feel better once every single trace of trauma was gone from his internal organs. Castiel placed two fingers to Sam's forehead and began the healing process.

Speaking of traces… This wasn't the first time Castiel had sensed something strange during their healing sessions, but he had thought perhaps it was a fluke, or the interference of the foreign grace swirling inside him that he was accessing for power. But now Castiel realized what he was sensing faded a fraction more each time he healed Sam. He frowned in contemplation.

"What?" Sam asked.

Castiel startled slightly. "Nothing."

Sam slapped his hand away. "You're a terrible liar."

"That is not true," Castiel protested vehemently. "I once deceived and betrayed both you and your brother."

That…probably shouldn't have been a character quality he tried to defend.

Sam shot him an exasperated glare. "Okay, that's not the point. Cas, what's wrong?"

He shifted uncomfortably, almost wishing he was a better liar. Sam was still sensitive about the whole Gadreel fiasco, and Castiel didn't want to agitate him further. But…Sam did have a right to know.

"I noticed something. It's, uh…it's resonating inside you."

Sam's brows rose a fraction. "What?"

"Something angelic."

Sam's throat bobbed, and he ducked his gaze a moment, obviously trying to collect himself. "Okay, uh, what the hell does that mean?"

Castiel grimaced. "Maybe we should call Dean." Whatever was inside Sam wasn't a threat to the Winchester, that much Castiel could tell, but he still thought that Dean should be there to help with this, since Castiel didn't know exactly what they were dealing with.

Sam shook his head sharply and moved to take a seat in a chair. "No. He wanted to go, and he's gone. We'll handle this."

Castiel held back a sigh. It made his heart ache to see the brothers fighting like this. Granted, what Dean had done was monumentally stupid and dangerous, and a lot of bad had happened because of it. But he had only been trying to help Sam because Dean loved him so much. Castiel understood what it was like to try to help, only to make things worse. He knew it all too well.

"Alright," he said. "We can start with research."

Research revealed that Sam had a remnant of Gadreel's grace inside him. Grace that they could use to track Gadreel if they extracted enough of it. Unfortunately, that involved a procedure that would be horrendously painful for Sam, not to mention the Men of Letters had never been able to test the tracking spell, so there was no guarantee it would even work. Sam was intent on it, though.

Castiel really thought Dean should be there for this, but if they called him, there was no way in hell he'd let his little brother torture himself this way. And while Castiel wasn't as motivated to go this route as Sam was, he had to acknowledge that there was potential value in it. Which was why he was following Sam toward the medical ward.

"Sam, may I ask you a question?"

Sam smirked. "You just did."

Castiel paused. Oh, right. "Can I ask you another question?"

"Well, technically, you…" Sam glanced over his shoulder, and then shook his head. "Yeah, go ahead. What's up?"

Castiel stepped into the doorway of the medical ward and stopped. "Sam, the Trials. You chose not to go through with them for a reason, didn't you? You chose to live rather than to sacrifice yourself. You and Dean…you chose each other."

Sam rolled his neck in obvious discomfort. "Yeah, I did. We did. And then…Dean made a choice for me." He pushed past Castiel into the medical room.

"What Dean did…it was only because he loves you so much."

Sam let out a derisive snort as he set the box he'd been carrying on a table next to a gurney. "Dean didn't trick me into angel possession because he cares about me. He did it because he's selfish. He…" Sam shook his head in frustration. "He's afraid to be alone, that's what it is. But you know what? It doesn't even matter what Dean did. I could have put a stop to all this, Cas. I could have closed the gates of Hell." The regret and self-loathing wafting off the young Winchester made Castiel's heart ache more.

"Oh, Sam." He wanted to offer some kind of comfort or encouragement. Certainly Dean would know what to say if he were here. Or…maybe not, given his role in the present circumstances. "You and Dean have fought before, and you've always found it in yourselves to forgive each other. Hell may be still be open, but you two are stronger together, as you've proven time and time again—"

"Dean's gone, okay?" Sam cut him off. "This is on me now, and if I can find Gadreel…I can fix this."

It took Castiel a moment to realize Sam wasn't talking about closing the gates of Hell anymore. That had been before Gadreel had even entered the picture. No, Sam must be talking about the prophet, Kevin. Of course he would blame himself for the boy's death. So did Dean, Castiel knew.

Sam unbuttoned his plaid shirt and shrugged it off, leaving just a thin white tee. "Now, being a human means settling your debts." He picked up the box again and passed it to Castiel before sitting on the gurney. "Let's start balancing the books."

Castiel opened the box and looked at the huge syringe. He knew what Sam was trying to do; Castiel had been trying to do penance ever since he'd taken Sam's Cage scars. But why couldn't the Winchester see that he didn't need to do this, that none of what happened was his fault? Gadreel's actions weren't on Sam. In fact, everything could be traced back to Castiel's foolishness and his own poor decisions. Gadreel never would have tricked Dean and Sam if the angels hadn't been cast out of Heaven. And if Castiel hadn't stupidly believed Metatron, he wouldn't have lost his grace, and would've been able to heal Sam himself. The young Winchester shouldn't be punished for Castiel's mistakes.

Unfortunately, the only way to begin to rectify them was to track down Gadreel, and this was their best lead.

Castiel lifted the syringe and moved over to the gurney where Sam was now laying. Positioning the four-inch needle at a seventy-five degree angle, Castiel carefully inserted it into Sam's neck, right below his ear. Sam let out a pained gasp.

Castiel winced in sympathy. "Now comes the part that will actually hurt. I'm gonna begin the extraction."

He slowly pulled back on the plunger, and a splash of angel grace spilled into the bottom of the barrel. Sam jerked, hands fisting at his sides as he bit his lip to keep from crying out.

"Is it working?" he wheezed.

"Yeah…" Castiel admitted, staring at the tiny bit of celestial essence.

Sam shuddered. "But?"

Castiel swallowed. "I-I need to push the needle in deeper. We need more grace in order to cast the spell."

"Okay." Sam gritted his teeth. "Do it."

He hesitated. "Sam, if I get too close to—"

"Dammit, Cas! Just do it."

Shaking his head in exasperation, Castiel nevertheless pushed the needle in deeper. When he pulled back on the plunger again, Sam's body lurched violently, and he suddenly made a series of choking hiccups. Castiel sensed Sam's cells quivering on the molecular level as they inexplicably began breaking down. He quickly withdrew the needle.

Sam's eyes snapped open, pupils wide and wild as he gasped for breaths. "What the hell was that?"

"Your body is regressing to the state it was in before Gadreel." Dammit, Castiel should have known this was a bad idea.

Sam tried to push himself up on one elbow. "Do we have enough grace for the summoning spell?"

Castiel shot him a disapproving look. "Sam."

"Do we or not, Cas?"

Castiel drew in a sharp breath through his nose. "No."

Sam laid his head back down. "Then keep going."

Castiel glanced between a shaky Sam and the tiny bit of grace in the syringe's vial. They shouldn't do this, yet they had no other method of finding Gadreel. Castiel reluctantly reinserted the needle into Sam's neck, pushing it deeper. When he pulled the plunger back again, Sam's body jerked soundlessly. The Winchester bit his lip against the pain, shaking almost uncontrollably now. Castiel had to brace a hand on Sam's forehead so he wouldn't accidentally torque the needle. More grace flowed into the vial, and then Sam went limp.

Castiel's heart jolted. "Sam? Sam!"

Sam's eyelids fluttered. "Keep going," he rasped.

"Why?" Castiel nearly growled. If they kept this up, Sam could very well die. Didn't he see that?

"We-we," Sam stuttered. "We have to find Gadreel."

"It's not worth your life."

"No," Sam gasped. "My life's not worth any more than anyone else's—not yours or Dean's…" His voice cracked. "Or Kevin's. Please. Please, help me do one thing right." Sam grabbed Castiel's hand and tried to make him push the needle in further. "Keep going."

Castiel was torn, he truly was. He hated to see Sam suffering, not just from physical pain, but also the emotional trauma and guilt of the past few months. Castiel could stop one, but increase the other. Or vice versa. And he hated that this decision was in his hands. But Sam's wishes had been made perfectly clear…

Steeling himself, Castiel pushed the needle in further, and Sam finally screamed.

Castiel tried to shove his reservations aside, tried to summon up the callousness he used to possess as an angel of the lord, the emotional detachment that let him make hard decisions for the greater good. But he couldn't do it. Sam, his friend, was lying on this gurney, suffering immeasurably as Castiel did nothing short of torture him. Greater good or not, Castiel wasn't that angel anymore. He didn't want to be.

"Hold on, this may pinch." He pulled the needle all the way out, and then pressed two fingers to Sam's forehead, healing the degeneration of his cells and the wounds received during the extraction process. Castiel had never been more relieved to see the blood that'd been dripping from Sam's nose and ear vanish.

Sam jerked upright. "Cas, what the hell was that?"

"I've healed your wounds completely," he replied casually.

Sam sputtered at him. "And the grace?"

"Well, whatever grace was inside you is gone now. What's left of Gadreel is in here." He set the syringe on the table. "We'll just have to try the spell with what we have."

Sam swung himself off the gurney. "Dammit, Cas!"

Castiel drew his shoulders back. "Sam, I want to find Gadreel as much as you do. But nothing is worth losing you."

Sam snatched his shirt up and stormed to the corner where he roughly shoved his arms through the sleeves.

Castiel sighed, deflating some. "You know, being human, it didn't just change my view of food. It changed my view of you. I mean, I can relate now to how you feel."

Sam turned around, quirking a perplexed look at him. "What are you talking about?"

Castiel smiled ruefully. "The only person who has screwed things up more consistently than you…is me. And now I know what that guilt feels like. And I know what it…" He swallowed hard. "I know what it means to feel sorry, Sam. I am sorry."

For so much, more than they had time to go over. Castiel could only pray the spell worked, and he could finally do something to perhaps ease both their guilt, even if Sam's wasn't rightfully his to bear.

Sam's expression softened, his reply quiet, "I know."

Castiel offered an encouraging smile, and picked up the syringe sloshing with grace. "Let's see if this works."

They headed back out to the war room where they'd gathered the supplies for the spell. Castiel mixed the initial ingredients in a bowl, and lastly plucked the plunger from the bottom of the syringe before tipping the glowing grace into the mixture. He said the incantation, and held his breath.

The ingredients in the bowl began smoking, and then popped with a startling sound. Tendrils of ash curled upward, and then bent sideways to trickle down over the map table. Castiel narrowed his eyes as it settled over a spot in Colorado.

"That's where Gadreel is," he said.

A muscle in Sam's jaw ticked. "Guess we're going on a road trip."

Castiel refrained from mentioning that's exactly what Dean and he had done the last time they'd tracked Gadreel—wearing Sam. But this time wasn't a rescue mission.

Castiel briefly considered calling Dean and giving him a heads-up. Perhaps the older Winchester was in the area and could join them. They could certainly use more hands on deck. But Sam would be angry, and Dean hadn't exactly been willing to stick around himself. And in that state, the two brothers were likely to throw each other off rather than work together as the skilled unit they could be.

Which left just Castiel and Sam to hunt down the infamous traitor of Heaven and Earth.