A weakened Castiel tries to avert the events of the apocalypse by stopping Azazel from tainting Sam, but time travel never seems to work out as expected. Stuck in the past, years before Dean is sent to Hell and Castiel is first ordered to retrieve him, the Angel's Grace tied irrevocably to his vessel Jimmy Novak, and with a young Claire in tow, Castiel must slowly make his way through time in the traditional manner to see if he managed to save the world, and see if he can woo a certain, stubborn Winchester along the way.
Castiel holds Dean’s hand as the stars go out. Once brilliant lights flickering, blinking, fading. Dying. Angels dying. Heaven dying.
Sam said yes.
Castiel looks down to the head in his lap, grip tightening on the cold fingers laced with his. Dean’s eyes are no longer windows to a soul long since departed, but glassy with unshed tears and cloudy in death.
Cas. Please. You’ve got to, before I—
Castiel cards the fingers of his free hand through his hunter’s hair, staining the fine strands with Dean’s own blood. He murmurs quietly to himself, voice wavering but refusing to break, “For you, Dean…? Anything. Even this.”
He leans down with a shaky sigh to kiss Dean’s forehead and whispers against his temple, Castiel’s lips brushing the clammy skin there as he speaks with a heartbreaking tenderness, “I’ll fix it, beloved.”
Castiel leans backwards until his wings are pressed against the cold earth; leans past it, sinking slowly through the ground. He keeps hold of Dean’s hand as long as he’s able. Their fingers part, and then he’s flying. Harder and faster than he has ever before, even with his failing Grace. He wraps time around himself like an old friend and throws everything he is into the past, searching out that one fixed point.
He will change it. He has to change it. Even if it kills him.