Disclaimer: I don't own Supernatural or its characters - these were created by Eric Kripke - I'm just borrowing them. I'm not making any commercial gain. No harm or infringement intended.
Cas is unable to do more than watch the Winchesters self-destruct until an author puts him back on the right path. Spoilers for season 7. Descriptions of Hell. Heavily implied pre-Dean/Cas
This story runs parallel to, and follows the events described in 'No Way Home' and 'The Devil on Your Shoulder' so I'd encourage you to read those first. It's another dark one, I'm afraid, but it's always darkest before the dawn.
Día de Castiel (Castiel's Day)
The universe is under the control of a loving purpose, and that in the struggle for righteousness man has cosmic companionship (angels). Behind the harsh appearance of the world there is a benign power – Martin Luther King, Jr
Día de los Angelitos
Ever since the Leviathans had fled Castiel in search of more controllable, numerous and edible prey, he had been lost. They had eaten their way out of him, and in doing so had seemed to sap some vital spark. It had taken weeks before he'd recovered any semblance of self and even now he wasn't able to take physical form, as close to a ghost as an angel was ever going to get.
So it was that, like a spirit, Castiel had been floating and drifting through the void, sometimes with periods of lucidity, but more often like a dream that he couldn't wake up from. He had suffered through a brief period of dreaming before, when he'd been cut off from the Host, just prior to the Apocalypse, and he hadn't liked it then either.
Angels, especially those of his pay grade, were used to dealing with moral certitudes and dreams are the very antithesis of this. He often wondered, as did many of his brethren, how humans made it through their odd little existence without going insane. Those like Raphael had not been at all convinced that they hadn't, and this dichotomy of God being a perfect being, yet creating such an imperfect species had led to no end of troubles as well documented in the Gospel of Winchester.
But Castiel had found that the longer he'd spent with Dean the more he'd come to believe that it was how they grappled with these uncertainties, especially when the deck was so heavily stacked against them, that was what made their father love humanity above all others. Castiel kept it to himself, but he couldn't help but wonder if it also made Him more exasperated with them too - the thought of being subjected to his father's wrath made him shiver with fear and feel grateful he was an angel and beneath His notice.
It was a sobering thought then, when he remembered how he had laid waste to Heaven under the influence of the purgatory souls. In some ways he found it even more disturbing that he could have ever forgotten that he'd committed such terrible deeds. While at the time it might have seemed like watching another person's actions, the result still meant that there was unlikely to be anyone in Heaven willing to hear his cries for help, although that didn't stop him from trying.
During his more coherent moments he had concentrated on trying to reach out to Sam, thinking that the young man's latent psychic abilities would make him an easy option, but there was just too much demonic-generated static - it was like they were radios operating on different frequencies.
A couple of times he'd got through to Dean, but to his horror all he'd achieved each time was to trigger a series of really quite unpleasant nightmares and repressed memories of Hell in his friend.
It really pained him to see how much guilt and grief Dean carried on his behalf. It both touched and frustrated him that Dean seemed to take some sort of personal responsibility for his actions and it filled him with shame to look back on those actions of the past year.
He realized that it was hubris that had made him want to strike out on his own. He had so wanted to visit Dean one day, when there was no emergency for once, and to be able to say, "Hey, Heaven tried to reboot the apocalypse, can you believe it? But don't worry I took care of it for you," and Dean would give him that crinkle-eyed grin and pat him on the shoulder, before throwing an arm around him, and tell him how pleased he was with him. Then they would celebrate with apple pie and beer and sit on the hood of the Impala looking up at the stars.
Cas sighed, Dean was right, I really am a baby in a trench coat. Still, it doesn't hurt to dream a little, he laughed at himself as he realized the torturous and contradictory loop his thoughts had just made.
He was distracted from his brooding when he caught a supernatural sense of the Winchesters and followed it out to an old, isolated cabin in Montana. He tried to materialize, but - as he expected and feared - whatever the Leviathans had done to him back at the reservoir still stopped him from taking physical form.
As he stood there, a silent presence, he reflected on how ironic it was that after all the times in the last couple of months that he'd done just this and been terrified of being found out, now he was desperate to be discovered.
The longer Cas watched the Winchesters the more his concern grew at an alarming rate. Sam was in the kitchen talking to an invisible presence and from the energy signature it was clear to Cas that it was Lucifer, or at least a portion of him that had somehow broken out of the Cage. Whatever it was, it wasn't quite strong enough yet to detect Cas' presence.
Cas looked on stunned as Sam continued to talk to Lucifer as if they were old friends, either not knowing, or caring, that this was the real deal and by acknowledging Lucifer he was giving him tacit permission to corrupt him further. Cas watched in revulsion at the roiling psychic clouds of dark smoke rising to envelope Sam's soul. You're inviting him in, Cas screamed, but of course there was no one able to hear him.
He remembered how desperate and afraid he'd been that the brothers were going to stop him in his attempts to access Purgatory. He'd been insane with worry that Raphael was going to win, and that as key players Sam and Dean would be the first casualties of the War for Heaven. He was the closest thing to an expert on the Winchesters and knew that something as 'small' as their own deaths wouldn't stop them from proceeding with a course of action.
To keep them distracted and off his back he'd smashed down the wall in Sam's mind that Death had created to shield Sam from the torment he'd suffered in Hell. Cas had never understood what had made him do what he'd done – it didn't make sense, he knew exactly how much Sam meant to Dean, he'd already given up Heaven for the man, and he would have been willing to sacrificed the earth for him without a second thought. But somehow when he'd looked at Sam at that point he'd needed to smash that wall. Was Lucifer there the whole time? Did I let him out?
He tried to contact Dean to warn him, but - as was usual these days - Dean was so inebriated that, even given the strength of their special bond, he couldn't do more than sense Dean's fragile mental state, how he was crumbling under the weight of his own guilt, a burden he seemed so willing to take up, and so unable to relinquish.
Cas was pleased to see Sam make the effort to try to comfort Dean, as it showed that at least the younger Winchester wasn't yet lost to them, but the adverse reaction of his handprint on Dean's shoulder to Sam's touch – trying to push him away - meant that the corrupting influence of the fallen angel had still grown very strong.
He watched Dean flee in emotional turmoil and managed to float along after him like some sort of ghostly puppy. He sat undetected in the passenger seat of the car, and although he'd never been fond of human transport, he missed the comforting sensation of the cold vinyl warming to his body heat, and the seating shifting and settling to his weight.
A couple of times he thought he caught Dean looking right at him out of the corner of his eye, but Dean just blinked, muttered under his breath and continued driving.
They arrived at a lake, Cas couldn't understand why they were stopping, but the waves of pain coming off of Dean were like a series of physical blows in their emotional intensity.
Cas was dismayed at how much Dean continued to drink, then horrified as he watched him stumble in his drunkenness and fall headlong into the lake. Dean had been standing on an overhanging ledge so his fall was into very deep water and the weight of his thick jacket and boots just dragged him down further.
"Hang on," Cas screamed in desperation, unmindful that as before there was still no one to hear him. He searched high and low in vain, but there was no moon to cast any light and the water was just too dark and murky, until he realized he was panicking and instead used his bond to pull himself towards his human charge.
He found Dean flailing as weak as a kitten as if he'd already given up, his lungs already full with water. Castiel reached for Dean and by some miracle was able to grasp a firm hold and pull him from the water, leaving just enough strength to drop him by the side of the lake and force the worst of the water from his lungs. He hovered with indecision over Dean, whose breathing was so shallow as to be almost undetectable, unsure which of the man's many injuries he should deal with first.
The decision was snatched from him as he no longer had the energy to hold himself together and, with a splitting pain that coursed like fire through his entire body, he exploded into his constituent atoms and was scattered by the wind.
His last conscious thought rang through the cosmos, Dean Winchester is saved.
There was a sensation of time passing, but it could have been a second or a millennium for all knew. As Castiel coalesced back to his familiar form he slumbered and dreamed terrible dreams of creatures made from teeth and retribution.