Struggling against the invisible bonds that held him in the chair, Dean felt waves of shame crashing over him. It had taken 3 weeks and 4 days for him to realize that it wasn't Cas riding in the backseat of the Impala. Sam had noticed. He had kept asking what was up with Cas and Dean had shrugged it off.

"Ah, the shame spiral. You are just so predictable, Dean." Lucifer purred, the deep baritone twisted in a way he should have recognized weeks ago. Dean looked up and searched the face of his best friend and now he couldn't find Cas there anywhere. The blue eyes were cold and foreign, no fondness or warmth. The laugh lines and worry lines that had appeared despite Cas' ability to stay forever young, had smoothed over and created a pinched almost false appearance.

Yeah, so Lucifer was right. He was shame spiraling. Because his best friend had willingly allowed Lucifer to take him as a vessel. And he had done it for Dean.

"Cas!" Dean yelled down the stairs of the bunker, slamming the door behind him.

Castiel jumped off the couch and ran towards the main entrance. No matter how common the sight, Dean would never get used to Cas in sweat pants and marathoning Netflix.

"Dean, what's wrong?" His eyes were full of concern as he checked Dean over for obvious injury.

"Sam is back in the Cage. With Lucifer. Cas we have to go in. We have to save him."

Cas took a step back and ran a hand through his messy hair. "Dean, my grace is still-"

"Cas," Dean pleaded. "It's Sam. Whatever it takes. We gotta go get him."

Castiel shook his head quietly and ran a tired, shaking hand over his face. Dean waited, watching his friend and realized just how tired he seemed. Cas stood up straight and nodded. "Of course. Whatever it takes."

Lucifer had been unwilling to let Sam leave without agreeing to be his vessel. Dean should have known something was up when eventually he caved and let all three of them go after many mind games but mostly unharmed. Dean had said whatever it takes.

And now his best friend, Castiel, Angel of the Lord, was playing Host to Lucifer. And it was entirely his fault.

"Dean, oh, Dean. Which part is haunting you the most? The fact that he did this for you or that you and I spent 3 weeks palling around and you had no idea Cas had left the building?" Lucifer's cold dead eyes brightened. "Or maybe you're feeling guilty about chasing evil tail while Cas was possessed by little old me?"

Dean's teeth ground together and he looked away as the accusations flew from his best friend's mouth. His neglect of Castiel had been epic over the past months. Hiding him away in the bunker with only Netflix for company for weeks at a time had been cruel. Dean hadn't called and he had been too busy with Amara to return Castiel's calls. The first time he'd gone home and actually seen Cas he'd basically told him to trade himself for his brother. And then he'd been too busy with Amara to notice Cas was gone. He should have noticed. If only because his grace was suddenly healed and he was no longer interested in TV. He was around all the time and he wasn't himself.

Dean hadn't noticed. It wasn't even him who had figured it out. It was after a romp with Amara that it had been pointed out to him how much he had changed since they had started up together. She had mocked him for turning his back on being the Righteous Man. She had then pointed out that Lucifer had been inhabiting Cas since their return from the pit.

"She is a spitfire, isn't she? But not, quite your type. Despite how hard you try to prove it to yourself and everyone else." Lucifer taunted, arms folded, eyebrow raised.

Dean's head snapped up and his eyes narrowed. "What the hell does that mean?"

Lucifer chuckled and the deep rasp was painful to hear. "You know, it's kind of fun in here. Your brother is a bit lanky, Castiel is-" Lucifer raised his arms and admired Cas' body. "More compact, agile, beautiful. Don't you think?"

Dean wasn't going there. Not with the fucking Devil, thank you very much.

"But maybe the best part is getting inside his head. Castiel has always been hard to read. He's an oddity. He really was a terrible angel. None of the other angels understood why he couldn't just follow orders. And now I know! It's like there's this puzzle that everyone has tried to solve and failed at! And only I have been able to solve it!" Lucifer grinned, eyes sparkling. "Oh, Dean! If only you could see what I see, know what I know. It truly is delicious."

Dean scowled up at the Devil wearing his best friend. He struggled futilely against Lucifer's bonds again but didn't budge.

"Should I tell you?" Lucifer wondered aloud. Tapping his chin he looked up at the ceiling and winked at him. "I want to keep you in the dark, but oh! I want to see the fallout! The Devil really is in the details, my boy."

"Go to Hell." Dean muttered, teeth clenched. His anxiety spiked. He knew what was coming and he just couldn't deal with it. He didn't think he could bear it. That must have shown on his face because Lucifer grinned as wide as a cat, reminding Dean hideously of the Leviathan.

"Dear Castiel has always loved Humanity. It was God's command and he took that to heart above all others. I don't know if you know this, Deany-boy, but Castiel was created for the purpose of saving the Righteous Man. Our Father brought him into existence knowing that he would be the one to pull you out of Hell, millions of years in the future." Lucifer leaned back against a table, crossed his feet at the ankle and let the news settle.

Dean was silent for several moments. He thought back over his first conversations with Cas and couldn't remember Cas ever having mentioned that.

"Didn't tell you, huh? So you see, Dear Cassie was invested in you millions of years before your birth. You were his purpose. It's a pretty big deal. Not many angels are created with that kind of responsibility. There are the Archangels, but Castiel is the only angel to ever be assigned to one human for one task. But God took off. So Heaven dictated what the assignment was. Castiel was meant to pull you from Hell and as he pieced you back together he was to impose Heaven's will on you. You would have woken up with a desire to serve Heaven above all else." Lucifer tapped his lips. "I don't think that was Dad's original plan. Castiel disobeyed. Just the first of many times for you. He brought you back and left you exactly as you had been."

Anger bloomed in Dean's chest. He was not going to have a heart to heart with Lucifer. Never going to happen. Especially when he was wearing Castiel like a condom.

"Get on with it."

"Did you know that your soul holds a piece of his Grace? It's the reason he has never died for good and the reason the Mark never took you fully. When he was healing you, your soul snatched a piece of his Grace. Unusual, isn't it? You've seen him heal others countless time. But it hasn't happened with anyone else." Lucifer chuckled. "Oh Denial is fun, isn't it? Well, maybe for you. While you've been off playing house with Lisa and Ben, Castiel fought a Civil War in secret so you could have your family. When he finally took your advice and found a woman she stabbed him to death. And when he needed you the most, you were sleeping with the female form of literal evil. My sins are child's play compared to Amara. But hey, your denial is working for you. And as long as Castiel is buried, he doesn't know you don't care about him." Lucifer finished coolly.

"Hey, fuck you. You don't know anything about me and Cas."

Lucifer crossed the room like a snake and leaned into Dean's space. "You're wrong. I'm in his head. And he's been in yours. Originally Castiel was my co-pilot, Dean. He's not as willful as your brother and he was willing to work with me to get rid of the Darkness." Lucifer's face twisted into a snarl. "But that lasted about 3 days. Because sharing space with that much misery and heartbreak was stifling. I physically couldn't breathe!" Lucifer punched the wall beside Dean's head, causing the drywall to crumble. "We're angels, Dean! We don't even need oxygen!"

Dean growled. "What did you do to Cas?" He yelled.

Lucifer laughed and wrapped his hand around Dean's throat. "What did I do? I put him out of his misery. He's curled up like a cat so deep in the recesses of his own mind that nothing can touch him anymore. What you did was fail to notice he was no longer there. He watched as you continued to fall further into The Darkness. The jealousy and the heartbreak were hard enough to bear. The fear that you were finally going to fall so far that even he couldn't save you was too much."

Dean's eyes were stinging but he laughed harshly. "It sounds almost like you care, Lucy."

Lucifer let go of his throat and stepped back, eyes still narrow. "I do care. Castiel, however misguided is an angel and he is my brother. You, disgusting mud monkey, are a waste of space and will never be good enough to deserve the loyalty he has given you freely and without question." Lucifer straightened his tie. "Angels feel emotions and everything else in a way that is indescribable to you puny humans. It would be like comparing a scratch to a slowly severed limb. So when I tell you that his heart is broken, I want you to picture a literal organ that is black and rotting, shredded, festering and burning with nerves that can never die, only short circuit and begin to hurt again."

The tears had begun to fall now, despite his anger and stubborn will.

"So yes, Dean Winchester. I'm angry and I care. Because I understand his pain, because I had to feel it." Lucifer sauntered forward and shot him a grin. "But hey, what's done is done, right? I'm sure you've learned your lesson. So I'm going to do what I promised and kill Amara and you're going to either help me or stay the hell out of my way. And then, if I let Castiel have his meat suit back, maybe you can quit with the drama and just man up." Lucifer shrugged and then ran a finger down Dean's cheek over the trail of a tear. "But then again. I really do love this body and Castiel is just so unhappy."

A deep growl snapped free from Dean's chest and he tried to lunge forward, held tight against the wall. "You son of a bitch! You give him back!"

Lucifer chuckled. "I'll make you a deal, Dean. Stay out of my way. Keep away from Amara and I'll think about letting Cas go."

Dean struggled furiously, red in the face. "What happened to all of that empathy, huh?"

Lucifer stopped and looked over his shoulder. "Oh, Dean. Someone needed to give you a good dressing down and I wasn't going to pass up that opportunity. But give me a break! I am the Devil after all." He winked and then he was gone.

The hold on Dean disappeared and he fell to his knees as Sam burst through the door. His brother was by his side before he'd even managed to look up.

"Dean!" Sam called out, when Dean didn't immediately acknowledge him. He shook his shoulder and finally Dean turned a blurry eye towards him. "Dean, what happened? I've been trying to get in for like 5 minutes but all of the doors and windows were just unbreakable. Didn't you hear me?"

Dean shook his head. "You were right, Sammy. Cas isn't okay. He isn't himself. It's Lucifer."

"No." Sam replied, shaking his head. "No way."

Dean's head hit the back wall with a thud as he blinked back more tears. "It's him. We had a nice little chat."

Sam exhaled sharply and sat back on his haunches. "Shit. What do we do now?"

Dean stared at the ceiling and made up his mind. "We let Lucifer do what he came to do. He kills Amara and then he said he might give Cas his body back."

Sam turned to look at him but Dean avoided his gaze. "That doesn't bother you? I know you and Amara have been-"

"It's Cas. There's no choice. I should've seen it sooner. He's in this mess because of me. So I'm gonna get him out of it. It's Cas."

Sam helped Dean to his feet and the two of them slowly left the motel room. A kind of understanding seemed to settle on Sam's face along with a long awaited relief. Dean ignored both as he drove away from the Motel and Amara and his mistakes. And for the first in a long time he found himself praying earnestly to a God he no longer believed in.