Dean's eyes widen as he sees the black substance slow down in its consumption of Castiel after having gotten rid of Billie. The goo slows down to a crawl, as if (no, surely) mocking Castiel's demise in front of his beloved.
Dean scrambles on his feet over to where Castiel remains rooted to the ground as the darkness envelopes his torso. He claws with desperation at the substance, hoping against all to save Castiel for once, or to at least reclaim a modicum of time with him. He fights for purchase on the black liquid, but grabs nothing.
"Cas, no," Dean says as his voice breaks into hoarse sobs.
"Dean, there's nothing you can do." Castiel looks down upon Dean, watching over him for the final time. His eyes are sincere and accepting.
And with that, Dean understands. Dean accepts. Dean resolves. Guard down.
He stands on his feet, trying to ignore the soft squelches the black goo makes as it creeps across Castiel. He brings himself to face Castiel directly and cups Castiel's face. If this is the last he saw of Cas, he never wants to forget those blue eyes.
"Cas, buddy," Dean speaks shakily. "We've tried our best, haven't we?"
Castiel returns a soft smile. "That we did, Dean."
"You tried. You've always tried to do your best. What you've done, it has always been to protect the ones you love, the ones you cherish."
"I only learned from the best, as you have already heard from me."
Dean chuckles at that and decides to accept that with grace this time. As he decides to make these last moments less painful for Castiel.
“The last 12 years, Cas. Ever since you stumbled your way into that barn, you – ” Dean says. “Ya know, they say change is the only constant. You have been my constant.”
For a moment, Dean sees a flash of fear in Castiel's eyes as his body tenses. He immediately wraps himself around Castiel. The black goo is immaterial, both there and not-there between Dean and Castiel.
"Don't be scared, Cas. It'll be fine. You'll be fine. I'll find you if that's the last thing I do," Dean whispers into Castiel's ears.
Dean pulls back and returns his gaze to Castiel. Time is short.
"Dean - I'm sorry to have put you in this position. And...I'm worried about -"
"You apologise for nothing, Cas. And don't you - don't you worry about me. I'll be fine."
"Promise me, Dean," Castiel's brows knit as he pleas. "Promise me, you'll carry on."
"What I can promise is that I promise to find you, one way or another."
"Dean - that's not -"
"And for what it's worth now, you have me, Cas," he feels tears prickling his eyes again. "You always have."
Neck. Adam's apple.
Dean reaches in for a peck and leaves a kiss on Castiel's lips for the first time. Maybe the last. He pulls back and Castiel looks at him, dazed.
"I love you, Castiel."
Cas breaks into a smile. That wide, gummy smile.
"I love you," he repeats.
"I love you."
"I love you."
Blue eyes on green.
"I love you."