Dean, Cas, Sam and Jack are all on a hunting trip, they stop for food(yes much like jacks heaven) and Cas is being Cas making Dean laugh while looking back on his baby Jack to make sure he's still there and behaving because let's face it that kid is danger prone.
Dean is just gazing (as he does) at CAS and he kinda just let's Cas talk. In the end he ends up just saying "Cas?" Cas looks, and tilts his head, ready for the question " you are such a good dad to him, I love that most about you. How much you care, how you care with everything in you. Never change." Then Cas gets a feeling in his chest akin to pressure. He sighs ready to tell Dean he loves him more than a brother and stronger than a friend.
He gets the feeling.
The wind blows taking away the comfortable warmth but not replacing it with heat. It's just nothing. And he looks at Dean and his nose how small and perfect it is. And looks at his lips well his smile. How that alone brightens any day.He stops and sees his eyes. How incredibly green they are. How they would look aganist the never ending blackness that is coming for him. Jacks feels it too and he goes to investigate. But sees what he normally sees Cas and Dean playing that game, the staring contest (who knows what they win).
Cas is just looking at Dean and trys so hard to memorize what he sees because he knows forever will be better with Dean in his head. The wind gets stronger and Dean finally pieces it together. His face goes from those cute crinkels around his eys to worry lines. Cas finally speaks, he looks past Dean to Sam "Sam, you take care of him, he still has so much to learn". Sam looks worried, not sure if he's referring to Dean or Jack. Jack walks to him but is stopped by Dean
"Jack, I wouldn't change a thing, nothing. You are MY son and no one can change that." Jacks pulling against Dean begging for him to let him go. Dean doesn't. He looks like he's in so much pain and Cas knows he has so little time before he's gone. "Dean Winchester, know that I finally found my unicorn. And he's everything I could ever pray for."
Cas lifts his arms ready for the empty Dean finally moves to him and says, "Cas?"
And just like that in a tearful blink he's gone.