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This day has been coming for a long time now. He's known it for a while, in fact, ever since he stole the grace burning away inside him. And today's the day it finally burns out.
He doesn't call Sam or Dean, because what's the point? He's of no use to either one of them anymore. Dean's a demon now, anyway, and Cas thinks he'd rather just bury himself in memories of his Dean. While it's true that even as a demon Dean's not entirely heartless, he's crueler than he used to be, and Cas just can't face that. Not today.
Cas decides to walk to the hospital. It's not really that far away and it'll give him a chance to have one final look at the world his Father created. Spring is in full swing now, plants blooming, birds chirping, and bees beginning to make honey. Everything is just so alive. There are worse days to be your last. As he walks he meets people, a sixty-five-year-old grandmother with arthritis and a killer coconut cake recipe, a twenty-six-year-old mechanic who bought his own motorcycle last year, a forty-year-old single mother of two who somehow manages to do it all. Cas smiles at them and lets them talk. It's been years since anyone talked to him like this, not since he and Dean stopped talking after what happened in the crypt. It feels nice. The grandmother gives Cas a hug. She doesn't have arthritis anymore.
He uses the badge Dean gave him one more time, and although it really shouldn't have worked, they let him through all the same. Somehow he's grateful to have been able to use it one last time.
In a different time and place, he'd heal them all. Chicken pox, colds, and cancer in one fell swoop, but today he has to be choosy. The small things will heal in time. Cas is looking for something that won't. He finds Mary while her mother is on a coffee run to the cafeteria. She's eleven years old, frail, and probably only has another week to live if he doesn't do something. If anyone's worth burning out for, it's her.
“I'm going to touch your forehead now. Don't be afraid.”
“I'm not,” Mary says, and smiles at him. How could he help but smile back?
It takes all his remaining strength to completely remove all traces of the disease consuming her body. When it's all done, he gives her one more smile, brushes the hair back from her forehead, and tucks her blankets in closer around her body.
Leaving the hospital takes most of his concentration as he tries not to topple over. He manages to make it to a bench in the park across the street as his vision starts to dim. Mary. Where does he know that name? When he remembers, he smiles. Mary is Dean's mother. He's not sure how he could forget. Now his breathing is labored. He's spent the last few months wondering what his heaven will be like. Cas is sure that Dean will be there.
When he opens his eyes again, he's lying on the roof of a barn in Pontiac, Illinois. He weeps.