It is the most painful thing he has ever done. And he's including rebelling against Heaven and slaying hundreds of his siblings on that list.
It's as simple as reaching forward and running a hand over each of their foreheads. He does Sam first, and before Dean can even say "What the hell, Cas," to his brother dropping to the ground, he too is unconscious.
Rewriting history in their heads was a lot more simple than he would have thought. But he's just filled in everything since 2008 with made-up hunts and excuses. No Apocalypse. No Lucifer. No Leviathans. Every death is accounted for with a simple hunting accident gone wrong.
After that he drags their bodies back to the Impala and dumps them inside. Straightens them - poses them, almost, like a serial killer would his victims - and locks the door behind him. He's left an idea in their minds, a hunt, something he thinks they should like. The angel runs one hand lovingly over the car's dark hood before he begins to make his way down the street. Without looking back, he clicks his fingers.
Dean wakes first, shaking his head and rubbing at his eyes, one hand reaching over to pat at his brother's face until he wakes up to push him off. They share a laugh and Sam groans, stretching and cracking his stiff neck. "What happened?" He asks, going to buckle his seatbelt.
"Must have dozed off," is Dean's response as he leans forward to switch on the radio. He grins as he starts the engine, caressing the dash at the sound of his baby's purr.
"Do I need to get the two of you a room," Sam teases, turning up the music. They're heading to some place called Delta Falls to hunt what Dean thinks might be a wendigo, but he's willing to bet money it's just a nest of vamps.
They pull out onto the road and start to drive, Dean singing along to an ACDC song with a grin on his face. "One hot angel, one cool devil," he croons, his green eyes flicking up to the review mirror.
Reflected in it on the street behind them is a man wearing a tattered overcoat. The man looks to the ground and pushes his hands into his pockets before turning and walking away. He's not sure why, but he thinks he might have met the man before. He gives the mirror a thoughtful frown before shrugging off the feeling and pressing his foot down to the accelerator. The sun is setting, his brother is by his side, they've got good tunes playing, and they're on their way to a hunt.
Life didn't get any better.
It's months later and he still hasn't been able to let go. Castiel still considers himself the guardian of the Winchester boys. He's been following them, watching them, protecting them from harm. More than once on hunts there have been moments where they should have died, but he's always stepped in and taken the hit, anything to protect them. Monsters can't hurt an angel nearly as well they can a fragile human, so he's really just sacrificing a few moments of pain for their guaranteed survival.
After the third time he's stepped in when a Winchester should be dead, Dean turns to Sammy and jokes, "Maybe we've got an angel watching over us." The words twist in his heart and hurt more than a blow to the stomach, but he doesn't say a word, doesn't reveal himself. The angel will stay forever loyal to the idea of keeping them safe.
The weeks pass slowly for him but probably breeze by for the two of them. They've eagerly thrown themselves into the hunting life again, even teaming up with other hunters. One of them is a doe-eyed woman who likes to kick ass and take names. Sam's taken quite a shine to her.
Dean says his brother still owes him a twenty for that wendigo they got a few months ago. He doesn't think Sam is going to pay up.
He watches, like a good guardian, and he keeps them safe.
It's been two years now since he cleared their memories. They're both moving on, getting married, creating lives. Sam's finally finished his law degree and is one of the finest attorneys Kansas has seen. He's bought a house and he and his partner are starting to look like they're seriously considering children - it was just the other day Sam brought home a baby name book.
Dean on the other hand has opened up his own little workshop and has a job as a mechanic. He spends most of his days covered in grease and grime, repairing old cars and motorbikes that have gone and died on their owners. The Impala has a place of honour out the back of the shop, where she lives out her weekdays quietly, anticipating weekends where the two brothers team up for 'one last hunt'. But it never is one last hunt because there's always another monster coming their way. Neither of them mind it, though. It's just the life. And nobody gets out of the life.
As he watches them find their own lives Castiel realises he has become obsolete. They no longer need their guardian. It pains him to watch them live and grow as much as it makes him happy. He has become an unnecessary shadow on their lives, and dare he say it, the cause of much of their troubles. If you have an angel stalking you there is no doubt something will notice.
One sunny Thursday afternoon the angel finds an vacant park and sits himself down in the exact center. The air is warm, a light breeze kissing his skin before tumbling its way through the trees. He takes a deep breath and holds his angel blade tight with one hand. He realises his hand is shaking, so he grasps the blade's handle with the other and plunges it into his abdomen without a second thought. A white light seems to explode from his vessel's body as his grace dies and burns away.
The body slumps and falls. Black wings are scorched into the ground either side of it.
Sam Winchester grins into the phone as he takes apart another shotgun to be cleaned. "Yes, Dean," he laughs, dropping the bolt to the table and shaking his head. His older brother has never stopped babying him, and right now he's receiving yet another lecture on the importance of checking the salt lines. He's not an idiot - he checks the salt in the doorway every night before going to bed, and the windows at least once a week.
"Daddy!" He hears a voice call, and he's forced to hang up on his brother and go investigate. His oldest, Robbie, is a bit of a wild child. The kid might only be four, but he's a handful and a half of trouble. Sam enters the living room to see his boy on the floor, grinning, with marker all over his arms. A crude devil's trap is drawn on his forearm. With a sigh he scoops him up, laughing and ruffling at the boy's light hair.
"Come on, kid. Time for you to have a bath." The boy tries to squirm away but he holds on tight, shaking his head and pushing him down into the bathtub despite the protests. "Robert John Winchester," he says warningly when there's yet another escape attempt. It still amazes him the boy can stay still long enough for a bath.
"Daddy, where's Mommy?" The boy mumbles into his neck afterwards. He seems to have relaxed in the hot water, less an excited bundle of energy and more a sleepy child. Sam strokes his hair gently, soothingly, as he tucks the boy into bed.
"She's putting Cassie to sleep. Don't worry, she'll be in to say goodnight soon. Or hey, do you want to go say goodnight to your sister?" The sleepy child is already gone, his eyes shut tight. He plants a kiss on the boy's head and flicks off the light switch, closing the door gently behind him. Their daughter, Castielle, was born just six months ago. She's every bit as beautiful as he could have hoped, his little angel. He's not sure where the name came from. He just knows when he saw her, big blue eyes and dark hair, the idea of naming her Joanna seemed ludicrous. She was Cassie Winchester.
Sam yawns and stretches his arms above his head, turning his head side to side. He's exhausted. He thinks it might be time to go to bed, even if he has left that gun on the table. It can be finished up in the morning. He makes his way to his room and starts to unbutton his shirt while trying to pull off his socks with his feet.
Down the hall he can hear the baby start to cry. He looks out his door just in time to see his wife walk upstairs, open the nursery door and peer in.
"Sam honey, is that you?"