"You're sure you won't come with us?" Dean asked wearily, staring into the vortex. It wobbled sickeningly like the surface of an oil slick, black and opaque.
Castiel looked away and shook his head.
"No Dean. I have to stay here." He looked back plaintively, willing Dean to understand. "I've failed my bretheren too many times. I can't...I have to make it right."
Dean looked at the ground and nodded. He understood how Castiel felt. He understood the guilt and the responsibility that weighed him down. But that didn't make the thought of never seeing Cas again any less painful.
"Besides," he continued, trying to fill the awkward silence, "A world without magic, without heaven or angels, it's likely that this vessel will disintegrate. The structural integrity has already been compromised, and whatever has been holding it together in this universe, there's no telling-"
"Yeah I get it!" Dean interrupted gruffly. Castiel shut his mouth abruptly. Dean ran a hand impatiently through his hair. "I know, I just...I'm gonna miss you."
For a moment their eyes met, and Dean was sure he could see the ache in his soul reflected back at him.
Sam cleared his throat awkwardly. Dean glanced over at his brother, standing next to the portal with a dufflebag. The bag contained everything they'd be taking with them to the other world, the one without magic. Without demons, without the supernatural. A world where the Mark of Cain would be inert, nothing but another battle scar.
"Let's go, Dean," Sam said quietly. He recognized that this was hard for his brother, but he was thrilled with the idea of leaving this universe behind. Finally, he wouldn't have to hunt, or be hunted, and he wouldn't have to feel guilty about it. He could rest. He'd have a normal life. And he'd have his brother. He was trying his best to curb his impatience but if Dean didn't hurry it up he was going to drag him through the damn portal by force.
"You'll take care of my baby, alright?" Dean asked, trying in vain to keep his voice from breaking. Castiel nodded solemnly.
"I shall guard it with my life." They clasped hands, and the moment drew out a few moments longer, before Dean had to break away or lose his nerve entirely. With a final look back at Castiel's stony expression, he walked side by side with his brother through the portal.