Thinking about a mafioso!cas AU where Cas is constantly dressed in perfectly fitted suits, being driven around in big black SUVs to go intimidate the fuck out of people who have slighted the family. He takes Dean with him everywhere, leading him with the loop of a leather and chain leash that ends in a collar around Dean's neck. Dean is always dressed in a pair of jeans and a t-shirt, and his hair and skin always look like they are meticulously cared for. But he never says a word. He might give Castiel a silent blow job in the back of the car and then stay on his knees in the footwell to keep Castiel's cock warm for the rest of the ride, or stand behind Castiel's chair silently no matter how tense the meeting is that Castiel is taking, even if Castiel takes out a weapon and shoots the person he's talking to in the knees. He never says a word and Castiel never lets go of his leash.
Nobody knows how Dean ended up entangled with Castiel like this, but they speculate. That Castiel bought and paid for him; saw him on the street somewhere and had him kidnapped; that Dean owed the family so much money that he agreed to become Castiel's pet instead of getting shot in the knees; that Dean's brother is a drug addict and he's the one that owes the family money and Dean took on his debt. Just like Dean is always silent, though, Castiel never answers any questions about him, deflecting them like water.
They don't know that Castiel found Dean, high and malnourished, when he bust down the door of one of the family's heroin dealers one day. That Dean's eyes were so big in his starved face, and looked so lost, and Castiel didn't shoot him but reached out a hand, to touch his face. Took him to one of his safe houses in the city. Visited him when he could, brought him candy bars and methadone and later borscht from the family's restaurant. That Castiel was the one that washed out his dirty hair and cleaned off the filth of the flop house and made his hair and his skin shine again. That Dean was vacant and silent for weeks after Castiel saved him and didn't start talking again until they fucked, and Castiel took him into the shower, in front of him, in his arms, afterwards, and he started to cry and beg Castiel not to make him go away.
So the next time he visited, Castiel brought the collar. And the leash. And walked out of the safehouse with Dean being led three steps behind him. And Dean has followed him ever since, and he might not ever talk when they are out in public but he has a perfect memory and he remembers everything that is said in every one of Castiel's meetings; every name, every face, every detail. Even so, he doesn't talk all that much when they are in private, either, except to beg for Castiel, who never denies him anything.