Some say the world will end in fire,

Some say in ice.

From what I've tasted of desire

I hold with those who favor fire.

But if it had to perish twice,

I think I know enough of hate

To say that for destruction ice

Is also great, and would suffice.

-"Fire and Ice" by Robert Frost

"Grab a pen. It's time to say goodbye."

The sad thing is, if it weren't for the fact Sam has to die, Dean thinks he probably wouldn't have even given his brother that much. He's not much for long goodbyes, never has been. Hell, when Sam said yes to Lucifer, they hadn't so much as said the word.

Just a "could you please not watch this?" and a pumped-up Sam and a "we're here, you sons of bitches! Come get it!"

Then a "it's okay, Dean. I got him."

And that was it.

This reminds him of that, he realizes. One of them possessed of a power too great to control. Both of them looking that power in the face and saying 'Screw you', albeit in different ways.

One of them about to die.

Dean thinks maybe he understands where Cain was coming from, now, at the end. He has to leave, has to go wherever Death deems him safe from hurting anyone; but to leave Sam here without him would be a Hell all its own for his brother.

It's a mercy killing.

Not that the Mark much cares. It's singing in his blood, in that deafening way it has been since Charlie's death. But the anticipation of Sam's death somehow makes it even stronger, and all Dean can see is blood. Worse, that he feels any sense of anticipation at all about this. Part of him is disgusted, recoils from the very idea, hollers into the phone for Sam to stay away.

But it's not the part of him that's in control. Not anymore.

The Mark reigns now.

And so the Mark tells Sam where they are, hangs up the cell, takes up a relaxed but alert stance in the middle of the floor of the little cantina, and waits.

Sam doesn't take long to get there. Dean knew he wouldn't. He wants to protest, wants to throw his brother out, wants him as far from Death as he can possibly manage.

"We need to talk," the Mark says instead.

And talk it does, spouting Sammy's worst fears out of Dean's mouth: you're a liability. I must live, and you must die. I am willing to throw your life away for the greater good.

"You traded my life."

Sam's words are more surprised than accusatory, and while the Mark keeps Dean's face impassive, he's screaming inside like he never did in Hell.

Sammy, no!

The Mark's words now are so similar to the ones he'd used as a demon that Dean is beginning to wonder whether it was the black eyes or the curse talking all those months ago. Terrifyingly, the logic is beginning to make some sense—the two of them have broken the universe more times than they've saved it, and maybe…just maybe it is time for them to hang up their spurs.

Dean's heart breaks when he thinks perhaps Heaven isn't so bad a place for Sammy after all.

"There is no other way, Sam. I'm sorry."

Before he quite registers the sound of a fist meeting flesh, the right side of his face is burning and he's taken a step to the side. He blinks, shocked, as he realizes Sam punched him.

He wants to laugh, shake his head and thank his brother for snapping him out of it, fold him into a hug and take him back to the bunker, forget this whole thing.

But anger is sizzling in his veins, clenching his fists, removing the bandana from his sliced hand.

"Good," the Mark says. "Good. Fight."

And then he and Sam are trading punches, like they haven't in a very long time; but it's not a fair match, because the Mark is so much stronger than his still-dangerous brother. He's—It's—slamming Sam down brutally, one, two, three times before the younger man holds his hands up, curled on the floor at Dean's feet, panting, "Okay, okay…t-that's enough."

The Mark allows him to take a step back, to listen. Dean has a sickening feeling it's only because the poisonous curse is enjoying the anticipation of the kill.

The thought makes his stomach churn.

"You'll never hear me say that you—the real you—is anything but good," Sammy gasps as he pushes himself to his knees, and Dean's angry screams inside his own head have turned into agonized wails. He can't do this, he can't.

You can, and you will. We're in this together, Dean, for the rest of time. You chose it.

"…have to be stopped," Sam is saying. "At any cost."

Dean's heart thumps painfully in his chest as he recognizes what he's seeing on his brother's face. It's not bitterness, anger, resentment, or even fear. Sam's hazel eyes are wide and tearful, and he looks for all the world like the kid who jumped into the Cage five years prior.

It's acceptance.

"I understand. Do it," Sam says; and at the exact moment Dean's soul shatters, the Mark screams its victory inside his head. It has not tasted the blood of a brother in centuries, and the feral sense of triumph makes Dean's head swim.

Death is handing him the scythe, and he's taking it but he doesn't want to but he can't stop and he hefts it, looks down into Sam's bloody face.

"Close your eyes," the Mark orders.

Sam doesn't, and Dean is assaulted with memory so vivid, so clear, it's like it's happening in front of him.

"Close your eyes, Sam," an eight-year-old Dean pulls his gun on the wild dog snarling as it tears toward them. The toddler buries his little face in his big brother's shirt without hesitation.

"Sammy, don't look," a fourteen year old, newly-minted hunter tries to hide the massive laceration running up his side. He knows it'll need stitches, and he'll be damned if he's gonna subject a ten-year-old to that much gore. Sam turns around and buries his nose in a book for the next hour.

The Mark tries again, and this time, some of Dean shows through too.

"Sammy, close your eyes." The plea seems to reach his brother, because Sam's face changes. That terrible acceptance slips to the background for a moment, and those hazel eyes blink rapidly.

"Wait," Sam says, and Dean feels his control solidify, just a notch.

He won't kill his brother. He can't. Not standing over him while Sam begs him to wait, please don't, Dean stop…

Perhaps, he tells the Mark, screeching its indignance in his ear; perhaps if Sam had been struck down in the midst of a fight, a brutal punch to the temple, instant death…the Mark may have had its fill of family blood. But now? Like this?

Dean will never. Could never.

"Take these," Sam says, reaching into a pocket and pulling out two smallish scraps of photo paper. Dean knows without looking what they are, and his heart twitches in his chest. "And one day, when you find your way back, let these be your guide." Sam's cheeks are wet with more than blood now, and Dean is almost blindsided by the vivid flashes of memory at that look on his little brother's face.

An elementary school auditorium, all the first graders lined up on stage, Sammy's amplified voice slightly muffled because he held the mic too close. "My hero is my big brother, Dean. He takes care of me and makes macaroni-and-cheese and sings me songs and when I fall down, Dean makes it better. He's the best brother in the world! My hero is my big brother, Dean." A jerky bow, indulgent applause, and the biggest swallow to hold back tears.

"They can help you remember what it was to be good…"

Heartbreak on a younger Sam's face, when he realizes Dean made a deal for his soul. "What do you think my job is? You save my life, over and over…don't you think I'd do the same for you?"

"…what it was to love."

"You're my big brother. I'd die for you."

And suddenly, all Dean can see is Sammy on his handlebars, Sammy shooting a gun for the first time—the kid was a crack shot from the start—Sammy curled up beside him on a ratty hotel bed, Sammy rounding the corner in the bunker looking half asleep, Sammy greeting him with a smile and a "So get this, Dean," Sammy's laugh when he pulls a particularly good prank, Sammy crying himself awake right after losing Jess, Sammy asking questions, punching his shoulder, laughing at his lame joke, driving his Baby, Sammy Sammy Sammy…

The pictures are lying on the dirty floor, and they bring tears to Dean's eyes. The Mark of Cain is still wreaking havoc on his insides, demanding he kill his brother, just do it already!—it hasn't come this far to fail on the cusp of victory.

Fratricide. It can practically taste it.

What would Dean's mom think, seeing him now?

"It's for family you must proceed, Dean," Death's voice is close, just behind him. "To be what you are, to become what you've become, is a stain on their memory."

Daddy's little soldier, gone rogue. Broken now, defective, faulty.

"Do it, or I will."

"I don't care. You're my brother, and I'm taking you home."

It clicks.

He looks down at Sam, but the words aren't for him.

"Forgive me."

Finally, Sam closes his eyes.

Muscles tense, the scythe swings, buries itself in flesh and bone. He can hear himself breathing, harsh and shallow and shaking.

And Death crumbles into dust before his eyes.

Dean reaches for Sam.