This could be seen as a counter part to my longer Samifer fic Intertwinning (which I haven't updated in ages, oops.) or as a stand alone, it doesn't really matter. Dean's point of view on how Sam said yes to Lucifer.

Dean tapped his fingers on the table nervously. This was the third time in the past ten days when Sam just disappeared into thin air without giving a plausible explanation. He just left a message, a curt one written on a receipt, paper tissue or whatever he had just found in his pockets as if he was in a hurry to leave and nothing could hold him back. He'd even stolen Baby a couple of times! That was before Dean had a serious talk with him about running away and taking your brother's car with you and thus making said brother think someone kidnapped both you and the car. After that, Sam found another way of vanishing but Dean couldn't figure out how, where and most importantly why Sam was running away.

Dean understood the importance of alone-time. He had been glad when Sam went away for the first, second time. It was a welcome distraction when everything got a little crowded and claustrophobic and Dean had no suspicion. He thought Sam would just drive a car, stop in a diner and get a disgusting mix of something green he would – with a great portion of euphemism – call food and then come back. But as far as Dean could say, Sam was up to something a lot more exciting than just driving around the country.

"Dean," Castiel laid his hand on Dean's shoulder in an attempt to reassure the human. He was concerned for his companion's wellbeing, but even more so, he was anxious to see where Sam had been disappearing. He couldn't track him because of the carvings he had inflicted upon the brothers' ribs himself and he didn't take Sam's vanishing half as calmly as Dean, not even at the beginning. "You have to confront him when he gets back. It can't go on forever."
"I know," Dean sighed. "It's just... Once I ask him, he'll get defensive and the little trust we have will go in vain."
"I know it's difficult. But you know that Sam is capable of everything if he has a good intention to justify it with." Castiel slipped his hand into Dean's palm and interwoven their fingers. "I know he probably means well. But then again, most of the people who destroy the world do."
"And they destroy themselves in the process," Dean nodded.
"Yes. You know, there's a reason why he's Lucifer's vessel. They have to be very much alike to blend together perfectly. An ordinary human cannot contain a power as strong as the Lightbringer." The angel made a few steps to face Dean to stress his next words. "Lucifer can be very persuasive and he knows Sam better than you could ever hope for. Once Sam's fallen for him, there's no going back."
"But why?" Dean snapped at Castiel. He knew the angel didn't deserve to be treated as an anger release tool but his frustration was getting the better of him. On the other hand, Dean also knew that Cas understood. He had no idea how he deserved him, but he wasn't going to complain. "I was supposed to be the perfect vessel for Michael and I hate the guy."
"That's because Michael is - and I believe I'm quoting you now - a dickhead. He never truly understood that everything comes with a price. He thought he was an exception after God expelled Lucifer from Heaven, and he didn't realise you might have other plans with your body." Castiel explained. He felt strange; it seemed as if he was defending Lucifer, betraying Heaven. But that was how it was and he acknowledged it for the first time now. "And he was too lazy to come and get you from Hell. That wasn't a smart move."
"Because I fell in love with you the moment I saw you?" Dean grinned. As much as he was afraid what Sam was doing, he couldn't deny himself a good old-fashioned flirtation.
"You hated me as far as I remember. Stabbed and shot me a couple of times." Castiel retorted and smiled. "A profound bond is created between an angel and a soul rescued from Hell. It hasn't happened many times, but the few it did, there was a deep connection. I had to embrace your essence very tight to raise you through all the guards of Hell. You were a treasured prisoner."
"And Michael knew all this and still sent you to get my sorry ass? That sounds like a really stupid thing to do."
"Well, as I said, Michael was so sure of his importance that he dismissed the possibility that you might say no to him." Castiel sighed, "You're avoiding the topic, Dean."
"Yeah? And what am I supposed to do? Call Sam? Don't you think I've tried that already?" Dean let go of Castiel's hand and made a few quick steps to fetch his phone lying on a table. He held it in both his hands and pretended to type. "Dear Sam, are you dating the devil?"
"You haven't tried to speak to him though," Castiel frowned slightly. Dean was astonished to see that, Cas was usually a ball of fluff and sunshine around him.
"Okay, I'll do that. Calm down, man." Dean snapped and threw the phone on his bed. "Let's go grab a beer, I'm going crazy from all this waiting."

"Look around you, Sam." Lucifer stood beside the younger Winchester on the roof of a tall building. The wind was howling around them, grabbing their clothes and pulling their hair. Sam's vision was limited by the auburn halo, but he could see just enough to get the point Lucifer was trying to make.
"The world is falling into pieces," he felt the need to say those words even if they were redundant.
"Yes." Lucifer nodded, just as unnecessarily.
"But why? This city was fine yesterday. I know," Sam whispered. His voice was barely audible, certainly not intelligible and yet Lucifer understood perfectly.
"You know it's not me," he mused into Sam's ear, too close for Sam to mistake the cold resonating from Lucifer for the air around.
"I know," Sam nodded and blushed. He didn't know why; he was past feeling shame or guilt over something he was born to do. Merge in with Lucifer, to the extreme of the word. But as right and pleasant as it felt, he was still not over how different it was to be with Lucifer compared to being with anyone else.
"It's a message for your brother," Lucifer went on with his lips still just a few breaths far from Sam's earlobe. "Michael still hasn't given up hope to wear him to our epic battle."
"You said you didn't want to fight Michael," Sam sounded hurt.
It moved Lucifer; he thought he knew Sam well enough after the time they spent together, but it turned out they could still surprise each other.
"I don't," Lucifer wrapped his arm around Sam's waist but it was still too little, not even remotely close to enough. "But has your big brother ever listened to you?"
"Dean's not like Michael," Sam protested weakly. He was sure about those words specifically, but he couldn't be about the entirety of the statement. Dean cared deeply about him - went to Hell for him - but did he listen?
"No, he's not," Lucifer admitted with a hint of remorse in his voice, "he would die for you and stand by you under all circumstances. But would he truly support you and what you stand for?"
"I don't... No." Sam bowed his head.

"He'd try to convince you to change your mind." Lucifer rested his chin against Sam's shoulder. He was now enveloping Sam tightly as if he feared he might lose him. "He doesn't trust you to make your own decisions, he thinks he has to protect you from your childishness."
"I'm not a child anymore," Sam said defensively.
"You're not. Does Dean know it?" Lucifer made a dramatic pause. Sam could not see his smug smile but he knew it was there. He was hundred percent sure of it and the thought of being so familiar with Lucifer petrified him. And yet, at the same time, he was thrilled, ecstatic to be so close to him. He understood then what the 'two halves made whole' meant. If the concept of soul-mates was real, Sam knew this was what it felt like to find your best counterpart. "But I know you're not a child, Sam. It was confirmed last night quite well."
"You're so cheesy," Sam snickered. They stood there for a while in a pleasant silence, holding onto each other as if it was the best and only thing to do.
"Is it really happening because of Dean?" Sam asked in a hushed voice as he watched the ruin and desolation beneath them.
"It's not because of him, it is for him. It's not his fault," Lucifer elaborated his previous statement. "But either way, it's not your fault. You're not to be blamed for all malice in the world."
"Did you just read my mind?" Sam meant it only half as a joke; part of him was actually considering the possibility of Lucifer being able to guess his thoughts.
"Yes and no." Lucifer shrugged while he was still draped all over Sam. "I know you, your feelings, fears and hopes as if they were my own."
"Shouldn't I be able to tell what you think as well, then?" Sam inquired.
"But you are, you're just too shy to say it." Lucifer teased and nibbled at Sam's ear.
"Am I?" Sam responded with the same voice. He disentangled himself from the cocoon of Lucifer and turned around to face the fallen angel. He cupped Lucifer's cheek. "You want me; you want to be inside me so bad it hurts you. And you don't think I'll refuse when you ask me to be yours again."
"As if they were your own," Lucifer smiled gently, "are you absolutely sure about this?"

"I've always been yours, haven't I?" Sam whispered and pulled Lucifer close for a deep, passionate kiss. The wind howled all around them, and if that wasn't corny enough, it started to rain. Sam would laugh if he weren't so occupied with Lucifer. The weather was far from perfect, Lucifer had the coldest skin Sam had ever felt in a living man and yet it seemed like he was going to burn.

"I've always been yours, but you were escaping me for quite some time," Lucifer breathed out when their lips parted. "Ready?"

"Like never before," Sam nodded.

"Do I have a permission to enter your body?" Lucifer asked again. Sam thought he was being overly dramatic, but maybe there was an exact formulation, something like a spell, that had to be pronounced.

"Yes."

It takes longer than Lucifer anticipated. He thought they'd just blend in immediately, embrace each other and be together like when you fasten a zip. It was more like a jigsaw puzzle though; there were lots of things they had to agree on if they didn't want to oppress one another. It would be easy if Lucifer wanted just the body – he could burn Sam down, expel him, throw him away. If he wants to keep the soul as well, he has to create some space for it.

Sam is actually astonished to see how little of his body's capacity he's been using up until now. When he sees Lucifer making a home in it – in him – it leaves him awestruck. As if there's always been the spare room for Lucifer – and it was there, wasn't it? He was born to be Lucifer's.

When they settle down at last, they look over the edge of the building.

Is it ever going to stop?

I can't stop it. I could stop everything, all life, if you wanted. But I can't stop this.

Why are they doing it?

Because they want me to burn again. But I won't let them, not now when I have you. I will never allow them to hurt you.

Could I… could I speak with Dean? For the last time?

The last time. He won't understand, Sam.

I've got to try.

Yes, you do.

"Dean," Castiel emerged out of nowhere just like he always did. Dean was slowly getting used to it, but it still annoyed him. Had the angel been just a tiny bit more human, he would think Cas was doing it on purpose.

"Cas," he muttered in response and took a swig of beer. He never truly understood why Cas always pronounced his name with such solemnity, but when he didn't find it hilarious, it seemed cute.

"You don't have to worry about what you're going to tell Sam anymore," the angel announced and sat down heavily on a bed, "he's just accepted Lucifer."

"What?!" Dean bolted upright with the speed of light. Even though he had come up with hundreds of explanations of what Sam had been up to and some of them did include Lucifer, he never truly believed Sam would say yes to him.

"It's all over the angel radio, as you call it," Cas sighed and bowed his head, "Michael is… raging. He's summoning an army and will soon march the Earth."

"That's fan-damn-tastic," Dean sneered.

"He's going to get you," Castiel stated it, flat, devoid of emotion. His voice was hollow as if he spoke about a problem very far away.

"I'm not going anywhere with that guy, whatever he has to say or offer." Dean said resolutely. "Are you with me?"

"Yes."

Dean almost jumped up into the air when he woke up to the sound of someone banging on the door. Castiel was on alert already and standing with Dean behind his back. The hunter blinked a couple of times and looked around.

"What is it?" He called as he stood up and joined Cas on the guard. They looked at each other and agreed wordlessly on advancing. They stood by the door when they got an answer.

"Dean, it's me."

"Well, nice try, Satan." Dean shouted. "What do you want?"

"No, it's really me. I wanted to talk to you, to explain." Sam knew it was a bad idea; he had known even when he asked Lucifer to take him to his brother. And yet he couldn't help himself. He thought he owed Dean at least the last goodbye.

"What's there to explain?" Dean fumed but he opened the door nonetheless. He had to admit he had expected to see Sam's face ripped into pieces like Lucifer's old vessel was, but Sam looked as healthy as ever, maybe even better. Dean couldn't deny a certain gleam that radiated off him and it made him worry even more. This isn't what Sam should look like.

"Hi, Cas," Sam smiled at the angel who nodded but didn't acknowledge his presence otherwise. It hurt both Sam and Lucifer.

Why is he like this?

He was too young when I was banished. I think he's heard too many stories from Michael. He thinks I broke Heaven and made Father flee.

Oh. But it's me he's seeing. Can't he tell a difference?

You're a traitor as well now, Sam. Didn't you know that?

I did. I just kind of hoped that at least Dean and Cas would still be the same.

Nobody is. But you do have me.

And you have me.

"Earth calls…whatever you are, man." Dean threw his arms into the air. "You barge in here and then you just stand here and stare?"

"It's me. We're both here," Sam stuttered. He hadn't realised just how difficult it would be to see Dean try hard to pretend he understood when he didn't.

"You mean you and your pal the devil." Dean remarked in his usually sarcastic tone he used when he was afraid or indecisive as to what to do next.

"Yes, Lucifer and me," Sam nodded and acted as if he repeated the exact same words Dean had said.

"And?"

What am I supposed to say?

I don't know. I told you they would be hostile. It's not even their fault.

I thought Dean would stand by me.

He doesn't believe it's you.

But it is.

"Well, you were looking for me." Sam said after a panic attack. He could sense Lucifer was sad for him and he wanted to prove him wrong. Dean would accept him!

"Yes; to tell you not to give your body to the devil." Dean bellowed. "God, Sam, why do you never listen to me?"

"Why do you never listen to me?!" Sam cried in response, "you're always so sure of your truth. I tried to explain that Lucifer wasn't the bad guy. Did you listen? No."

"He's the devil! For god's sake, Sam, are you really that blind?"

"Well, I love him!" Sam roared. There was silence after that, both in and out as Sam realised. He had never said those words to Lucifer before.

You're damned. But I love you too, whatever will come.

Dean and Cas stared at him as if he had grown a pair of wings. Well, technically he did, but still. He hadn't done anything wrong and neither did Lucifer.

"Why should you get love and I couldn't?" He asked, his anger already dissipated.

"I don't have Cas inside me, do I? We're just normal people," Dean shook his head. He grabbed Castiel's hand roughly and raised it for Sam to see. "We can hold hands, look."

"Am I supposed to be impressed?" Sam shrieked, unable to tell how he should react to that.

"No. You're supposed to see that what you have with Lucifer is sick. He's inside you, Sam. Doesn't that strike you as unnatural?" This was the first time Castiel had spoken since Sam came into the room.

"I'm sorry for coming and ruining your afternoon together," Sam mellowed. "Your kind of love is obviously the only love you can understand. Good-bye, Dean."

"Wait, Sam… is he still there?" Dean stepped closer to Sam but farther than he would have before.

"He was. He was speaking to you the whole time," Lucifer replied as Sam backed away. He felt no need at all to speak anymore and Lucifer understood that. Being hurt by your older brother when you expected him to protect you was not an alien concept to him. "But you hurt him. He doesn't want to talk to you anymore."

"It was… truly him? But how?" Dean gasped and glimpsed at Cas who shook his head. "I thought you kill your vessel."

"Angels do. Most archangels would. It's easier to just obliterate the soul, I admit." Lucifer made a face Dean had never seen Sam do. "But I need Sam by my side just like I need his body."

"Sammy," Dean whispered and stared into the eyes that had belonged to his brother, "Sammy, come back, please. We'll figure a way how to get you rid of him."

Tell him he had a chance to keep his brother. Maybe later I might forgive him. But right now it just hurts.

I know it does. I can feel your pain.

As if it were your own?

Yes.

"Sam says goodbye. And that you had a chance to keep him but you hurt him." Lucifer looked at Dean with disdain. And he thought he had finally found a big brother who wouldn't give up.

"I didn't know it was him," Dean defended himself.

"He told you, Dean, don't you remember?" Lucifer said without the slightest hint of interest in the hunter. "Don't go looking for us. I warn you. If Sam wants to talk to you, we'll find you. But don't bother him, don't hurt him again. I won't let you."

Tell him I'm happy with you.

He wouldn't believe me.

Let's just leave then.

They turned around together like one man. It wasn't obvious which of them gave the command to their legs; they both wanted to be as far from Dean as possible. They slammed the door behind them quite ungracefully.

"We've got to find them and get Sam back." was the first thing Dean said after he woke up. He wasn't getting too much sleep lately and he always rose up with the same thought crossing his mind.

"Mm-hmmph," Cas mumbled from slumber and secured his grip on Dean's naked chest.

"Cas, wake up!" Dean shook with the angel. He did enjoy the warmth of his lover all over him, but they had a quest to go on. Yes, the sex was awesome and Dean loved every single pant, whimper and touch of it but they couldn't do it all the time; Cas's surprisingly experienced moves would have to wait.

"Five minutes," Cas begged. Dean found it adorable, he could stare at his sleepy angel for a long time.

"We don't have five minutes," he muttered, irritated. He wanted to get from below Castiel, but he soon realised it won't be so easy. Maybe it was his grace but the angel felt as heavy as a dead body.

"Cas, you weigh tons," Dean groaned but he smiled as he planted a soft kiss on the crown of Cas's head.

"You liked those tons all over and inside you yesterday," Cas grinned and finally opened his otherworldly blue eyes. He fixed them on Dean for a while before he reached forward to peck Dean's lips.

"Yeah, but we've got work to do," Dean said harshly.

"Dean, he doesn't want to be found, and Lucifer's no idiot. The only time you find him is when he allows it." Castiel propped himself up on his forearms beside Dean.

"He's a monster like any other and I hunt monsters for living." Dean claimed with his badass voice.

"You know he's no monster. And you know Sam doesn't want to see you." Castiel murmured gently. "This crusade of yours is useless."

"Is it?" Dean snarled and stood up. Castiel could see all the scars, bruises and marks covering Dean's body he didn't have time to notice the night before. "Too bad it's the only thing I can do."

"You can fight, you can help destroy the virus that has spread over the world-"

"Oh yeah, the virus your lovely brothers let out into the void and lost the antidote for!" Dean shouted at Castiel. The angel rose to his feet in a swift, elegant motion, grabbed Dean's shoulders and pressed him against the wall.

"I lost everything for you! I abandoned my family and friends to be with you. Don't you dare make me lose you too." Castiel's eyes hid thunderstorms and an abyss of hopelessness. Dean got lost in them for a while.

"I'm sorry, Cas," he whispered and escaped Castiel's embrace. The angel watched him walk away and he realised for the first time that morning how tired he was. He needed to sleep now, he needed to eat and he noticed his body got injured more easily than before.

Why didn't they stop it by now? They must know he's not going to accept Michael.

Maybe they're still hopeful. Though I'd bet they don't know how to stop it. That would be quite typical.

Why is Dean looking for us again?

Castiel's become human. Dean is worried as to what's going to happen next.

Does he still think you caused all this?

Wouldn't you?

If it weren't you, of course.

He wants to kill us now. He stopped believing you're here with me.

I won't let either of you kill the other.

I'm afraid one of us has to die, Sam. There'll be no peace for us as long as one lives. I'm sorry.

Don't be. It won't happen, I won't allow it.

At the end, there was nothing Sam could do to stop Dean from dying. And when he looked at it more rationally, there was no reason to do so. Dean was fully aware of what was expecting him when he stormed into the building. Lucifer had said it was a suicide mission and Sam believed it when he saw Dean's eyes – dead, hopeless, ruthless. There was only a shell of his brother left; less than a trace of humanity in his heart because the Dean Sam had known would never walk into a trap like this with people at his side. He led the people he loved into certain death and Sam's brother would never do that even if the price was a chance to kill the devil. Especially not if it meant killing Sam.

Sam closed himself into a ball of sadness and depression and he didn't let Lucifer in for weeks. That way, he never knew about Lucifer's conversation with Dean from the other dimension and time, the Dean that felt lost forever. He didn't see the smug, self-centered side of Lucifer that got him expelled from Heaven. But even if he did, he wouldn't have cared. Lucifer deserved Heaven and he was getting close to getting it.

With all the angels fleeing, losing their graces or just stepping down from a fight almost as terrible as the first millennia ago, it left just Michael and few of his most loyal and most stupid minions to stop Lucifer and Hell's army. And Lucifer had one thing Michael never had and never will have – his true vessel's allegiance and unconditional love. If there was anything that would decide the war, it would be the power of their joined entities. As an old proverb would say, the whole was better than a sum of its pieces.