"Cas?" Sam said at last, still staring down at his struggling brother. "Let him up."

"What?" Cas spat, looking up at him for the first time. Cain stood some distance away, the box containing First Blade held tightly in his hands. "Are you insane?"

"Let him up, Cas," Sam repeated, not looking at him.

Cas scoffed.

"Cas. Let. Him. Up."

Cas shook his head in disbelief and tightened his grip on the demon's throat for a moment before relenting. He stood up in one fluid motion and took a small step away from Dean.

"Get up, Dean," Sam ordered, his voice carrying an authority he rarely used.

Dean's scowl was enough to make a lesser man quail. Rubbing his jaw, his eyes flicked back to an achingly familiar green as he rose to his feet, breathing heavily, his broken wrist hanging gingerly at his side.

He glared at Cain, fuming. He saw the locked cursebox held in the old demon's arms and his expression darkened.

"I knew you were a murderer and a failure; I didn't know you were a traitor too," he snarled.

Cain didn't respond, but his grip tightened minutely on the cursebox as the weapon inside clinked feebly against its cage.

"And you." He groaned, turning to Sam. "I thought I'd dealt with you."

Sam kept his expression cool and carefully composed. "Well, next time finish the job."

"I will!" Dean spat as he leapt forward, grabbing Sam by the throat and pushing him against the wall, crushing his airway. Cas was by his side in an instant, trying to pull the demon off of Sam. Cain was tugging on Dean's other arm.

"No – stop!" Sam gasped. Cas tugged harder, slowly loosening the demon's grip. "C-Cas! Stop! Leave him!"

Cas stared wide-eyed at Sam's grimacing face. "ARE YOU NUTS?" he shouted, all patience gone. He snaked his arm around the demon's neck and pulled, hard. He still wouldn't break his hold on Sam's.

"Cas! Cain! Let him go!"

Cas exchanged an incredulous glance with Cain.

"You heard the kid," the young demon snarled, a vicious smile curving his lips. "Let. Go."

Reluctantly, not quite believing his own trust in Sam Winchester, Castiel let go and took one, small, step back. Cain copied him, bringing his other hand down to hold the cursebox.

Dean shuffled closer, tightening his grip on his brother's throat. "Bold move, kid," he said in a low voice, looking down at Sam. He blinked and his eyes returned to deepest black. "Now what's to stop me breaking your neck like a toothpick?"

Finally finding his footing, Sam slowly let go of his brother's wrist, surrendering his meager defense. "Only – one – thing," he gasped.

Dean chuckled. "Yeah? And what's that?"

Sam swallowed against Dean's hand and felt it loosen just enough to allow him to speak. "I'm your brother."

Dean snorted as his eyes switched back to green. "If you think I still give a fu –"

"I'm your brother," Sam repeated, cutting across him. "And the last thing you would ever do, Dean, is hurt me."

Dean's laugh filled the atrium. "Then what was it I did last time? In that kip of a church?"

"You're just lost, Dean," Sam continued, as though there had been no interruption. "But it's okay. It's okay, Dean. I'm here. I've got you. I'm not gonna let you down again, big brother. I'm gonna get you back –"

The last word was contorted by Dean's tightening fist. Cas took a half step forward but Sam raised his hand slightly, silently ordering him to stay back.

"There's something you're not understanding, Sam," Dean growled, his voice low and dangerous. "I don't want to 'come back'. I like this" – his eyes blinked black – "I like being free!"

"No, Dean. You don't. You –"

The fingers tightened further. "You know nothing, Sam!"

"I'm not Sam," Sam wheezed. "I'm Sammy, Dean. I'm your Sammy." He felt tears pushing behind his eyes, and for once he didn't hold them back. "I'm your brother, Dean." The tears swelled in his eyes. He placed his hand gently, non-threateningly, on Dean's chest. "And I know I let you down, Dean." His voice faltered slightly but he kept going. He needed to say this. Almost as much as Dean needed to hear it. "And I can't tell you how sorry I am for that. I know how much you've lost. I know how much easier it must be to not feel." He took a quivering breath. "You're the best person I know, Dean. The best. Better than this."

Dean's brow was creased slightly as he stared at Sam.

"I need you, Dean," he said simply as the tears began cascading down his cheeks. "I need you to come back. I just want my big brother back, Dean, the one who'd let me have the last of the Lucky Charms. The one who stole Christmas presents for me, who drove me to the ER on his handlebars, who –" His voice cracked. He took as deep a breath as he could and reached his free hand into his trouser pocket, fumbling for his last hope. "The one who has always been there for me, no matter the cost, and I know how high the cost has been for you, Dean," he choked. "I know how much you've sacrificed for me. You've been looking out for me since you were four. All my life. You brought me back from the edge, back from Lucifer, back from the dead. You saved my soul. I want my big brother back, Dean. Please. I need you."

Sam pulled the long-forgotten amulet from his pocket. Slowly, he separated the thong and placed it carefully around Dean's neck. Dean looked down at the small horned head as it thudded into place against his chest. He blinked as he stared at the tiny golden figure, his brow furrowed. Slowly, his hand shaking, Dean released Sam's throat as he looked up at him. He looked so young and so lost. Sam felt his heart break.

He wanted nothing more than to pull his hurting brother into a tight hug, but he knew he was dealing with trust as brittle as spun glass.

"Will you come with me, Dean?"

Looking as though he wasn't quite sure where he was or what was happening, Dean nodded uncertainly.

Sam gently laid a hand on Dean's shoulder, nudging him toward the door that would lead them to the dungeon and the waiting Devil's Trap.

As Dean took an uncertain step forwards, the First Blade jerked suddenly in the cursebox, the frantic rattle breaking the silence. Dean's posture straightened just as quickly, and Sam saw his eyes turn black.

His eyes widening in sudden fear, Sam called out. "Cas!"

Before the angel could reach the demon, the back of Dean's hand slammed into Sam's jaw, sending him flying and crashing painfully into a wall of books. With heavy tomes raining down on his back and head, Sam struggled to his feet.

Cas was wrestling with Dean as he fought, snarling and spitting, to get to Sam. Cain's eyes were black as he held tight onto the cursebox that was jerking and bucking in his arms as the Blade tried desperately to return to its younger master. The locked lid jumped and bent as the Blade tried to cut itself free. Cain slammed the box to the ground, pressing both his hands over the lid with all the strength he could muster.

Sam pushed himself up and ran to help the old demon, ignoring his brother's howls of rage as he stepped out of reach. He called over his shoulder, telling Cas to get Dean to the Trap now.

Cain's face was contorted in rage and effort as he held on to the fiercely struggling cursebox. Sam fell to his knees and skidded to a halt. Ignoring Cain, he pressed his hand to the front of the cursebox and frantically invoked the strongest containment spell he knew. Silence fell inside the black box as the final word was spoken. Both man and demon sagged in relief. Sam brushed a hand through his hair, taking a deep breath.

"I never thought it possible," Cain muttered, sounding bewildered.

"What?"

"The First Blade ignoring me like that. Choosing another so passionately. Well," he added a little more brightly, meeting Sam's gaze. "I guess that's just payback for the Mariana Trench."

Despite himself, Sam chuckled. "Thank you," he said to the old demon, and he surprised himself by meaning it. "I'm not sure how I would've gotten Dean here without you. I, uh, I appreciate it."

Cain bowed his head once, accepting the praise. "You honour me, Mr. Winchester."

"Sam."

"Sam," Cain repeated, smiling slightly. There was an odd sort of energy in his eyes that Sam found mesmerizing. Must be the demon power. "That was impressive, breaking through to him like that. I can see why he chose to save you."

"Heh, yeah, well. That's what you did for your brother, isn't it? Saved him?"

Cain blinked in surprise and the energy lessened. "Yes. Although most people usually focus on the whole 'murder' aspect."

"Well, it does kinda stick out in history."

Cain nodded – sadly, Sam thought. After a brief pause, he looked up at Sam again, his eyes piercing. "I owe you an apology, Sam. I regret giving Dean my Mark. My curse. I know what grief it has caused you, and I'm sorry. I thought Dean would be able to control it as I do, without giving in to it first."

Sam nodded. "It's okay. Frankly, if you hadn't have given him the Mark, he'd probably have done something else to kill Abaddon, and I doubt any of those choices would've been any better."

"I did wonder why you let him take it from me?" Cain asked carefully.

"I, uh ..." Sam avoided his gaze. "I hadn't seen him in a while. He didn't exactly ask me."

"You were fighting?" Cain guessed.

"Yeah. A big one."

"Abel and I fought like rutting bucks. The only break I got from his arguing was when Seth joined in." An affectionate smile tugged at Cain's lips. Sam thought the action looked quite rusty.

A furious howl echoed through the hallway and both men looked in the direction of Dean's scream.

"Do you really think you can cure him?" Cain asked, his gaze searching Sam.

"Yes."

"You seem awfully certain."

"Well ... either I'll cure him, or I'll kill him. But if he dies ... at least he dies human. Or," he amended, "part human."

"I understand that," Cain said softly. Sam looked at him and, not for the first time, wondered what it must have felt like, murdering his own brother to save him. Now he might be about to do the same thing.

"Our families are weird," Sam sighed.

"I'm sorry."

"It's not your fault."

"Not entirely. But had my family been better, been stronger, much would have been different."

Sam puzzled. "What do you mean?"

"I embraced the Mark's power once Abel was dead. Can you imagine killing Dean – back before this happened, when you were younger? It's abhorrent. Unthinkable. Seth couldn't even look at me afterwards. He disowned me. He vowed his descendants would curse my name for all time. I was cursed anyway; he just didn't know it yet. We were all cursed, really. Ever since Lucifer came into the Garden, we were doomed. Evil ran in the family."

"What do you mean?"

"My mother. You didn't think she just took a bite of one apple and killed a few animals for their meat and hide, did you? The Bible gets more wrong than it does right. Eve destroyed the entire Garden. She gave evil a physical form. She created the first monsters. By the time God banished her and her ruined Eden – Purgatory – she had already created the first of all the species you've spent your life hunting. She had a lot to answer for."

Sam suddenly felt acutely awkward. "Do you know that's she's, uh –"

"Dead?" Cain finished. "Yes. To be honest ... I did not mourn her.

"I've done a lot of bad, Sam," he continued after a moment. "I hope that by helping you bring your brother back, I can at least redeem myself from the curse I placed upon your family."

Sam smiled. "You have. Thank you. To be honest, I'd thought you just told Crowley you'd help so he'd leave you alone."

"I never break a promise. Besides, if he was right about Dean attacking Heaven …" Cain's eyes darkened dangerously. "I would not allow that to happen. My love resides there. And my brother. I would die before allowing them to be harmed again." Cain's frown eased after a tense moment and he looked around. "Where is Crowley, anyway? I assumed he'd be here."

"He's dead. Dean killed him."

"Ah."

"He died saving my life."

"Crowley?"

Sam nodded.

"Well. Who knew the King of Hell could be so noble."

Sam smiled at that. It faded quickly. "I need to go to Dean. Are you ... gonna stay?"

Cain shook his head quickly. "No. No, I have ... business to attend to. I'll leave you to yours."

As he turned to leave, Cain scooped up the small black box. "I'll take care of the old bone. Good luck, Sam Winchester. I hope you save your brother."

"Thanks, Cain. I'm sorry you had to save yours the way you did."

With a last nod of understanding and a small, sad smile, Cain turned and left the Bunker. Once the door had clanged shut behind the ancient demon, Sam heaved a sigh as he heard Cas call his name.

Now for the hard part.