A missing scene from 5.18 Point of No Return. It might not be 100% true to canon, but it's at least 99% true to canon.

"Van Nuys? The beautiful room is in Van Nuys?" Dean asked in incredulity. He continued ranting. Castiel wasn't listening, except to provide to occasional affirmation that it was, in fact, in Van Nuys.

Castiel already knew where this ended for him: blown away by a sigil. Sam and Dean didn't, or they would try to create some other stupid plan that might end up with them dead. Castiel wasn't even sure they had the assurance of resurrection anymore. It wouldn't be nice and neat, at least- they'd wake up somewhere very bad. Out of the question.

Cas yanked off his tie. "There's at least five angels in there." He began to outline a plan, ignoring the Winchester's feeble attempts to stop him.

"That's suicide!"

"Yes, but then I won't have to watch YOU FAIL." Castiel pulled out an exacto knife.

"Hold up," Dean exclaimed, "you're going to take on five angels with an exacto knife?"

"I have a plan," Cas insisted.

"Cas," Sam cut in. "Tell us the plan."

"I will carve a sigil on my chest. If I can get all the angels to circle me, I can take them all out."

"Cas! That's suicide!" Dean exclaimed again. "You do realize you'll be blown away too?"

"The thought occurred to me, yes," Cas said indifferently. (Or, attempting indifference.)

"I'll go instead," Dean immediately said, but Cas shook his head.

"You could die. And if YOU die, you'll most likely wake up in a dream torture chamber, or worse. The angels are desperate."

Cas was fumbling at his shirt buttons now, trying to bare his chest. He'd gotten three buttons down when Dean took pity on him and helped him get the rest.

"What kind of angel are you?" Dean laughed mirthlessly. "Defeated by a few buttons."

"I have been on Earth a very short amount of time, and I rarely need to remove the buttons," Cas explained.

"There you are," Dean said, unbuttoning the final button. Cas peeled his shirt apart. Sam was doing his best to peer through cracks in the door, looking for any sign of angels.

Cas took the exacto knife and tried to cut into himself, looking down his chest. Dean snatched the knife from him.

"Isn't there any other way?"

"No," Cas said. "This is why I did not tell you the plan."

"Then I'll do it," Dean said resolutely. He scooped one arm behind Cas's back to hold him up, and began to draw. He tried to go as quickly as possible. Cas winced, but didn't cry out.

Dean redid the buttons on Cas's shirt for him, then unexpectedly pulled Cas into a hug. "You come back, you hear me? After you blow them all away, you come right back here."

"Dean..."

"Just promise me, Cas. Promise me you'll come back."

"...okay. I promise."

Dean released Cas, and let him walk through the warehouse door alone.

Meh.

Message from the lovely writer: Sometimes, stories pop into your head so long-forgotten that you've changed the plot entirely.