There are tears in Cas's eyes.
"I love you." It's everything Dean's ever wanted to hear, but not here, not now, not like this.
"Don't do this, Cas," he pleads.
"Goodbye, Dean."
Cas's shove sends him sprawling to the concrete floor, and Dean can only watch in horror as the wall explodes in black, tar-like goop, engulfing Cas in it's oil-slick embrace — Billie too, who has just broken through the door, but Dean barely notices, eyes locked on Cas's beaming, tear-streaked face as the Empty creeps across it.
"Me too," he wants to say, wants to at least give Cas that, but he can't make his mouth move.
The black goo swallows Cas and then lights up like a beacon, the blinding blue-white light of Cas's grace illuminating the storeroom dungeon with the intensity of a small sun. Still, Dean doesn't — can't — look away. He owes Cas that much, a silent witness to his last moments on earth.
When the light retreats at last, Dean has to blink rapidly to clear his vision. He lurches forward before he can fully see, expecting to find the print of charred wings across the brick and concrete. Instead, a figure stands there, swaying on it's feet.
Before Dean can react, Cas — because it is Cas — collapses, going down in a heap like a marionette with cut strings, landing with a heavy thud.
"Cas!" Dean scrambles forward now, going down on his knees before Cas's sprawled form, one hand grasping instinctively at his shoulder and one cupping a roughly-stubbled cheek. "Cas, are you in there?"
To Dean's relief, those familiar, beloved blue eyes flutter open a crack, squinting even against the dim light of the bunker's basement.
"Dean?" Cas groans. His brow furrows.
"Hey," Dean breathes, something loosening in his chest. "There you are." He attempts a shaky smile.
Cas doesn't return it. He closes his eyes again instead, wincing almost imperceptibly as he does. "Yes, it does seem that I'm… here."
Dean's too attuned to him to not notice the wince, or the laboured breathing. "You okay? All in one piece?" He's already sliding his hands over Cas's arm and chest, checking for injuries.
Cas catches the hand on his chest by the wrist, his grip weak, far weaker than an angel's should be. Dean stops anyway, his palm resting over the thrum of Cas's heart. It's racing.
"That was some speech you made," Dean jokes feebly, not knowing quite what to say, but desperate to say something, to acknowledge his feelings in kind. "You know I'm not one for words, and all, but, uh, I guess I owe you a ring, huh?" He tries to smile at Cas and ignore his alarm at the sickly pallor that has swept over him. He hadn't meant to jump straight to marriage, but here he is. Though, in hindsight, as proposals go, Dean could've chosen a better moment.
Or maybe, he realizes with a lurch, no moment would be good, and he's misjudged the sutuation entirely. Far from looking like a man in love, Cas just looks horrified. He breathes out a low, but surprisingly firm rejection, "Dean, no," before promptly passing out.