Summary: The Mark of Cain was changing him and he knew that. Would it change him so much that he would actually kill Sam if given the chance?

Setting: Between 9.17, "Mother's Little Helper" and 9.18, "Meta Fiction"

Warnings: Mild language. Mild violence. Brotherly fluff. Season 9 and slightly Season 6 spoilers. Kind of AU but also kind of just a tag to the two episodes, mostly 9.17. Direct quotes are used from 9.11, "First Born".

Disclaimer: I own nothing. Everything you recognize belongs to everyone and everything associated with Supernatural. I am gaining nothing from this piece of writing.

Author's Note: I took liberties with the whole Abaddon harvesting souls thing in this in regards to Sam. Review please :)

Just Fight

His arm burned and he rubbed at it absently, feeling the raised skin that was the Mark of Cain. The brand was warm under his touch and glancing at it showed how it glowed an angry red. He tugged his sleeve down again in that gesture that was becoming so common. He attempted to ignore the burn, the need, the urge, the ache that the Mark was flooding his body with, trying to focus on the book and articles he had in front of him. He tried to focus on his need to find Abaddon but with those thoughts came the reminder that finding Abaddon would reunite him with the Blade.

He closed his eyes against the rush that filled his body at the mere thought of holding the First Blade again. It was always in the back of his mind, pushing its way forward to remind him of what it had felt like to hold the Blade, reconnect the Blade and the Mark.

It was an obsession now, an addiction, and, while it thrilled him, it terrified him at the same time. He wanted to feel that power again but was scared of what that power could do, what he could do with that power.

The Mark was changing him and he was afraid there was no way to stop it.

He shook his head and shoved his fingers through his hair, scrubbed his hands over his face.

The Mark continued to burn and only continued to get worse.

"Dean?"

He dropped his hands from his face, one coming to settle on the handle of his knife sitting on the table next to his books. He didn't respond to his brother's call. His eyes fell to the knife he was slowly turning over and over.

"Dean?"

The Mark burned harder. He watched the light from the nearby lamp glint off the blade. It was disappointment he felt as it connected with his mind that this blade was not the First Blade.

"There you are."

He almost gasped at the violent increase in the burning coming from the Mark. It felt like his arm was on fire. Anger came from nowhere and flowed through his body, only thriving with the presence of the Mark and its innate desire to be reconnected with its other half, with the Blade. He tried not to think about why the feelings had gotten worse once Sam entered the room.

"You've been in here for hours," Sam said. "You haven't eaten all day. Want any supper?"

"No," Dean said shortly, still staring at the knife.

He rolled his head, feeling his tense neck crack. He swallowed thickly, feeling the effects of the Mark fill him.

"Dude, come on," Sam pushed. "You can't starve yourself."

"I said no," Dean said. His fingers clenched around the knife's handle.

"Dean..."

"Back off, Sam," Dean demanded, holding the knife tightly and not even trying to dismiss the wish that it was the First Blade.

"Dean, come on, man."

And then Sam's hand connected with his shoulder and the Mark pulsed. Rage, hatred, and drive exploded in his body and he found himself out of the chair and swinging around. He watched through a narrow field of vision as the blade of the knife he held disappeared into Sam's abdomen. His jaw was clenched so hard it should've hurt but he didn't feel anything beyond the rage, the bloodlust, and the almost happy pulse coming from the Mark of Cain.

His eyes slid up from the knife now embedded in his brother's body to meet Sam's eyes. Looking into the wide, shock-filled hazel eyes broke whatever hold the Mark had had on him.

He felt his own eyes widen as he realized what had just happened, what he'd just done. His mouth opened but no words came out. His hands were out, hovering around Sam's body but not touching him. Horror filled him when he noticed the blood on his right hand...Sam's blood.

Sam went to his knees, pain in his face, his hands circling the blade in his stomach. His eyes never left Dean, though, and Dean could not figure out why he didn't see anger or hate or accusation in his brother's gaze. There was shock and pain and what Dean could swear was resignation. Despite what had happened there was also love and Dean could not understand it.

"Sam..." he finally gasped out just as Sam tilted to the side and hit the floor. His eyes were blank now and wide as they stared at nothing.

His little brother was gone...gone by Dean's own hand.

"The mark can be transferred to someone who's worthy." Cain's words echoed in his head as he stared down at Sam.

"You mean a killer like you?"

"Yes."

Cain had been right. He had just proven it.

He was a killer and the perfect one to bear Cain's mark because he had just killed.

He had just killed his baby brother.

And the Mark on his arm continued to burn and pulse with pleasure.

Dean flew up in his bed, breathing hard and sweating heavily. It took longer than it should for it to connect with his brain that it had all been a dream. It didn't help that the Mark of Cain was burning on his arm but not nearly as bad as it had been in the dream. It was instead the common dull burn that was always happening daily.

He ran a hand down his face, trying to calm his heart. His phone going off on the bedside table made him jump. He snatched his phone off the table and glanced at the caller ID.

Sammy.

Yeah, he had Sam as 'Sammy' in his phone. It didn't matter what happened—Lucifer takes over, partners or brothers, 'I wouldn't save you'—Sam would always be his little brother and would always be 'Sammy'...even if he didn't actually call Sam that anymore.

He frowned at himself as he came to that realization.

The phone rang again and he hit the screen, answering it.

"Sam?" he said.

"If only it was," a familiar female voice said from the other end. Dean tensed and his fingers clenched around the phone.

"Where's Sam?"

"He's fine," Abaddon said, "for the most part."

"What do you want?"

"I'm keeping myself amused," Abaddon said. "The two of you are my current entertainment."

"What do you want?" Dean repeated.

"I want you to come to Milton, Illinois," Abaddon said. "You know, the place you let your little Sammy go all alone. It'll be more fun if you're both here."

"What will be more fun?"

Abaddon gave a small laugh. "Guess you'll have to come here and see. Wouldn't want to ruin the surprise."

"Let me talk to him," Dean demanded.

"Why would I do that?" Abaddon said with mock confusion. "From what I've heard, you two don't quite...connect the way you used to. Must be all the lies and tricks and...anger. Emotions are so...messy, aren't they? It's a wonder humans get anything done with them. The burden of a soul, I suppose."

Dean felt his body go cold at her words, a bad feeling settling in his chest and stomach.

"See you soon, dear," Abaddon said and Dean cringed when he heard a scream before the call was ended.

With Sam's scream echoing in his mind, Dean shoved his phone in his pocket and snatched his jacket. Sam had the Impala but when had anything so mundane ever stopped him from getting to his little brother before?

"I'm here," Dean said, pulling over to the side of the street.

"Didn't do your homework?" Abaddon said. "That doesn't sound like the big brother Dean Winchester I've heard so much about."

"Where are you, you bitch?" Dean growled into the phone.

"Temper," she said and did something that made Sam yell out again.

"Don't touch him," Dean spat.

"Too late for that, Dean," Abaddon said, making Sam yell again and Dean's body thrummed with anxiety the way it always did when Sam was being threatened or hurt. "I've had to keep Sammy interested while we've waited for you. We've held off on the real fun for your benefit."

"So kind of you," Dean drawled. "Now where the hell are you?"

"Come join us in the basement of the St. Bonaventure Convent," Abaddon said. "I think Sammy will be very happy to see you despite your recent differences, won't you, Sammy?"

"Dean!" Sam screamed and Dean closed his eyes, swallowing thickly at the deep pain the cry inflicted on him.

"Come quickly, Dean," Abaddon said. "Little brother misses you."

The call was cut off again and Dean slammed his fist against the steering wheel of the black sedan he had hotwired. He threw it into drive after searching for the convent on his phone's GPS. He flew through the small town and barely had the car stopped and in park by the time he was jumping out and running into the convent. He pulled his gun from the back of his pants despite the fact that it would be useless against the Knight of Hell and headed into the basement.

He entered the basement and moved quietly, searching for his brother. He was peeking around a corner when he found himself flying through the air to hit a wall. His gun fell from his hand and he groaned at the impact, raising his eyes to look hatefully at Abaddon.

She stood next to the chair that Sam was bound to, leaning on the back of it casually. Sam's head was lolling around as he tried to stay conscious. His white dress shirt that was part of his fed suit had been torn open and his entire torso was covered in varying degrees of cuts. Many were quite deep and bleeding sluggishly. His chest was covered in blood and his face was pale, revealing how much blood he had lost.

"Sam," Dean said, wanting his brother to show some sign that he was alright. "Sam!"

Abaddon ran a finger across Sam's cheek, leaving behind a line of red. Dean's jaw clenched and the Mark of Cain began to burn, reminiscent of his recent dream.

"Glad you could join us," Abaddon said and grabbed Sam's chin, forcing his head up to face Dean. "Say hello to our guest, Sammy."

Dean tried to struggle but couldn't even move a finger against the Demon power keeping him pinned to the wall. He watched as Sam's eyes fluttered and eventually opened, instantly spotting his big brother.

"De..." Sam breathed, his voice sounding rough and hoarse from screaming. It made Dean wince.

"I'm here, Sam," Dean said. "Feeling okay?"

He knew Sam had rolled his eyes the best he could and gave a huff that was supposed to be an amused snort. "Perfect," he muttered and Dean smirked.

"That's m'boy, Sammy," Dean said quietly and couldn't imagine being happier when he saw Sam attempt a smile.

"How sweet," Abaddon said with a sickly smile, patting Sam's cheek as she released his face. "Looks like there's still some brotherly love between you two. I think I can fix that, with the help of you two, of course."

Dean frowned at her.

"I've heard some very interesting stories about the two of you, including one involving little Sammy's soul," Abaddon said and Dean's stomach clenched as he realized where this could be going. "I like the sound of Soulless Sammy and would love to experience it. How about you?" She looked at Dean. "Missing Soulless Sammy?"

"Not even a little bit," Dean bit out. "Don't you dare touch him or I'll kill you."

Abaddon's lips curled into a disconcerting smile. "I don't think it'll be me you'll kill, not if that special mark of yours has any power."

The Mark pulsed on Dean's arm, filling him with its effects just like in his dream. He felt the need to hold the First Blade, felt the need to draw blood. He felt the rage and hatred. He felt his blood boil. The Mark burned and he knew it was glowing red.

Abaddon was smiling as she realized what he was fighting. "It has you." She walked over to him, swaying slightly to make the movement seductive and sultry. "You feel its hold, its power. You've held the Blade and you want it again." She slowly circled him, letting her hand run across his chest, over his shoulders, and across his back. "I can't give you the Blade but I can give you that rush. I can give you a fix, hold you over until you can have that Blade in your hand again."

Her lips were brushing against his ear, her voice falling to a whisper. Her hand stroked over his chest. He tensed and his eyes closed at the thought of feeling that rush of power again.

"I'll give you a little...nudge," she said and left him, returning to Sam's side again. Drowning in the Mark and its effects and his growing addiction, Dean just watched as Abaddon thrust her hand into Sam's chest.

Sam threw his head back and screamed as he felt the Demon's hand inside him, moving around. He knew what was happening and it terrified him. It also terrified him to see the expression on his brother's face. Dean was losing the fight with the Mark of Cain, was losing himself to it. He felt tears streaking his face, leaving behind hot trails but he couldn't find it in him to care when the pain was more than anything else he had experienced outside of Lucifer's Cage.

And then it ended. The pain was gone...but so was everything else. He felt...nothing. It was almost freeing but something was telling him it wasn't right. He looked around and found Abaddon watching him with an almost excited smile and Dean was just staring at him. He felt his bonds come loose and he looked back at Abaddon.

"What a machine you are," she said, sounding almost awed. "Why did they ever return this to you?" She held up an obscenely bright ball of glowing light. It was oddly pure in some way and it entranced Sam.

He looked away from it and to Abaddon when she moved over to Dean. He watched as she whispered to him, breathing something into his ear that made Dean's face hardened more and more with each passing second and word. Then the force holding him disappeared and Dean was free, his eyes stuck on Sam.

"Does Soulless Sammy have faith in Big Brother?" Abaddon said, slinking over to stand near the both of them. "Big Brother's got a lot of power running through him, power that's always been focused on one thing." She moved behind Sam, her lips touching his ear. "Murder. Death. The killing of a brother."

"Dean," Sam said, his tone monotone. Dean didn't so much as twitch. He looked down when he felt something being pressed into his hand. He looked questioningly at the dagger now sitting in his palm.

"Can Big Brother fight the brand that destines him to kill his brother?" Abaddon said, moving over to press a dagger into Dean's hand as well.

Sam's eyebrows came together. "He won't kill me."

"Maybe not you with a soul but you're currently Soulless Sammy, Dean's favourite," Abaddon said, smirking. She held up Sam's soul again. "You're not precious Sammy without this."

Sam swallowed, feeling a pressure in his chest that he supposed would be fear if he had his soul. "Dean, fight it. I know you can."

"Maybe I don't want to," Dean said and lunged forward. Sam reacted instantly, dodging and swinging his own dagger in defence. He spun and grabbed Dean, flinging him as far as he could before charging at Abaddon. Not expecting the attack, he managed to knock her over, making her release his soul.

He fought her as she tried to get away, watching as his soul fluttered around the room now that it had been set free. She managed to kick him off and he wasn't able to recover before Dean was on top of him. Dean knocked the knife out of Sam's hand, leaving Sam to be unable to do anything except grab Dean's arms to keep the knife away from him.

The ball of light that was Sam's soul headed towards him and, with trying to fight off Dean, was unable to do anything as it drifted into his open mouth. He shouted out in surprise and slight pain, not expecting it to actually hurt. But then it was in place and everything came rushing back.

Fear was the first thing he felt, both for himself and Dean. Then he felt concern, overwhelming concern for his brother. Then he felt anger at Abaddon and at Dean.

"Dean. Dean, listen to me!" he said, still trying to hold back his brother who had clearly gained extra strength through the Mark. "Dean, please!"

Dean didn't hear a thing, not really. The words entered his ears and connected with his brain but just enough to turn the sounds into actual words. Any meaning behind them was lost. All he was aware of was the Mark, the urge, the need, the absolute desire and drive to kill. The Mark wanted blood, wanted death and he wanted that rush, that power. He had to have it. So he fought this opponent, struggled against the limbs trying to hold him back but that was the key word: 'trying'. He was stronger than this enemy and he proved it in a quick move that brought his knife down towards the man's abdomen.

It had almost made contact when he stopped. Something made him stop. There was something telling him that this wasn't right. An image floated through his mind, an image of Sam sinking to the floor with a knife in his abdomen.

"You mean a killer like you?"

"Yes."

His eyes drifted up and met the hazel that he had known all his life. He saw the concern and the anger in the eyes. He saw the fear, a sight he had always tried to keep from his brother's face.

"Dean."

The feelings of rage, hatred, power, and bloodlust immediately disappeared even though the Mark burned angrily on his arm. The Mark burned, wanting him to continue, to finish what it demanded, to give it the blood of the brother it desired.

He couldn't do it. The Mark of Cain could do anything to him, make him do anything, but he would never allow it to make him hurt Sam and there was no way he would ever kill his brother. He noticed the knife in his hand and tossed it away in horror, pushing himself away from Sam.

"Hm, so strong."

Dean and Sam looked at Abaddon sharply.

"That strength will be useless in this, however," she said. She walked over to Dean who glared at her, shaking from both the after effects of what he had just felt and fear at what he had almost done. "You know what the Mark demands and an addict can only fight so long. The Mark demands it." She cast a glance at Sam which Dean followed. "And one day you'll give in. You're not as strong as you think because only someone worthy could bear that Mark."

She leaned in close and brushed her lips against his ear and cheek.

"I will enjoy it when you slit his throat, just like when Cain slit Abel's. A pity I wasn't there for that. It would've been quite the show but I suppose you two will do just as well," she said. "Perhaps better because you fight so hard."

Dean jerked away, clenching his teeth hard to prevent speaking.

She pulled back and smirked at him.

"Come see me when you've got the First Blade," she said. "I refuse to miss this destiny."

"Destiny means nothing to us," Sam spat, glowering at the Demon.

She turned her smirk on him. "We'll see."

And then she was gone.

It was silent for a long while, the brothers sitting on the floor as they processed what they had been through. Dean buried his face in his hands, unable to believe that he had almost killed his little brother. He had almost made his dream come true. He dropped his one arm so he could stare at the Mark of Cain. It was changing him, he knew that, but would it change him so much that he would kill Sam without a thought, without remorse?

He flinched violently when a hand settled on his arm, over the Mark. He looked up and found his eyes meeting Sam's. He opened his mouth to speak but nothing came out, not that he had any idea what he wanted or meant to say. It obviously didn't matter because Sam was pulling him into a tight hug, burying his own face in Dean's shoulder.

The contact and unexpected affection broke something in Dean and he found himself clutching desperately at the back of Sam's shirt, hiding his face in Sam's shoulder as well. Tears poured from his eyes as everything set in from the past several months; tricking Sam, fighting with Sam, taking the Mark of Cain, holding the First Blade, fighting against the Mark, fighting against his own body, the dreams of losing to the Mark, the dreams of killing Sam, the seconds old experience of almost murdering Sam.

"We'll get through this."

He realized then that Sam was talking and probably had been for a while.

"I'll help you, even if you don't want me to. I won't let this thing take you. It's just a mark. You can fight it, I know you can, just please try with me. Please. I know you have been but I know it's getting harder. Just keep trying, yeah? Keep trying."

Dean just clutched at his little brother tighter. He knew he should respond, reassure Sam that, yes, he would continue to fight, would continue to fight until he couldn't anymore for whatever reason. He couldn't speak, though, and just held onto Sam.

"Just keep fighting. I'll help you fight. You are stronger than this. You are stronger than Cain because you are not him," Sam continued. "We've kicked destiny's ass before. Let's do it again."

Dean swallowed thickly.

"We'll do it again," he managed to get out even though it was muffled against Sam's shoulder.

He knew Sam was smiling when his little brother spoke. "We'll do it again," his brother repeated back to him.

They fell silent again, just holding each other, comforting each other as they repeated the promise to themselves.

Dean held Sam tight, certain above everything else that the Mark of Cain might completely change everything about him but he would never...never... kill his baby brother.

The End