It started when Castiel cut himself shaving him one morning, and it didn't heal. That was the first sign. He didn't say anything to Dean, though, for he knew the hunter would only blame himself and try to find a way to stop it. But there was no stopping it; of that he was sure. And he knew that when Dean failed to save his grace, the guilt would devour him.
It got slowly worse, as he was wounded in fights and had to spend long periods of time away from the Winchesters as he recovered. He'd had to walk to the local pharmacy to get some painkillers for his injuries because his 'teleportation' powers were weakened – he'd tried to make the simple jump from Boston to Philadelphia, but found himself stuck on the New York subway and greatly disorientated. Suddenly surrounded by what could only be described as a herd of humans, he'd tried to call Sam but could get no service. He'd been left with no option but to follow them until he found his way out, and he vowed never to go there again. He'd taken a few extra pills that night, and found they helped him sleep better. And that was another thing – when did he start getting tired?
Then one day Dean nearly died when he was attacked by a demon that Castiel hadn't seen. He had seen the body but not the demon within, so had dismissed it as an ordinary human. Along with this, his ability to tune into the 'angel radio', as it had been nicknamed, also went. He turned to drink, as he had seen Dean do on numerous occasions, and it numbed his pain – though only for a little while. But then he'd drink some more.
Eventually he couldn't hide it any more, and as the months passed he found himself less useful and more of a burden to his friends, so he upped his dosage of pills and the amount of drink he washed them down with. It got so bad that he'd wake up with no recollection of the days previously, and he longed for the days that Dean would laugh at his naivety for the disappointment written all over his face made him feel even infinitely worse.
Sometimes he'd wake up next to an attractive young woman, or sometimes two. Other times it was a good-looking man with green eyes, but never Dean. He suspected Dean had had feelings for him once, but it was a long time ago. He'd missed his chance, there. Most mornings, however, he woke up alone.
Sometimes he'd forget to shave for days in a row, only realising he was growing a beard when Dean would say, "Nice peach fuzz." But his words weren't filled with warmth as they had been in Purgatory; instead they were cold and detached.
It wasn't until he walked in on Dean fucking a blue-eyed brunette, however, that he realised how far he'd fallen. Clearly Dean still felt something for him, if only on a subconscious level, but there was no way he'd ever act on it now. Castiel was a disgrace. His grace had burned out long ago, and if Sam and Dean left him behind one day he'd be nothing more than another dishevelled bum on the streets.
And so, with Dean and the unnamed woman still grunting and groaning in the next room as the headboard knocked repeatedly off the too-thin walls, Castiel swallowed every last pill and drank every last drop of booze that he had. When he lay down on the bed and closed his eyes, it almost felt like he was flying again.
Tears fell from his eyes, and he remarked with bitterness that it was the first time he'd cried. He mumbled a silent prayer to his Father, begging for His forgiveness; not knowing if Heaven still felt His absence but knowing that he'd never find out.
The noises from next door were getting more desperate, now, and he wondered who'd win in the race to the finish – them, or him. He could still pick out Dean's satisfied grunt under the high-pitched scream of the woman beneath him as they climaxed, so attuned was he still to the human he once saw himself as the guardian of. And as his vision blurred and the light began to fade, he could hear the door open and close as she left, and the bedsprings creak next door as Dean settled down for the night.
He imagined he could hear Dean whispering his name one last time as the darkness laid claim to his soulless body.