SPN AU where Sam’s demon powers start manifesting a lot earlier, when he’s a kid, right after he learns about the supernatural. And he freaks out at first, but then he decides that this can be good, he can use these powers to keep Dean and John safe when they go hunting. So he practices, and he gets better, he uses his visions, his telekinesis, all of it, to protect Dean and John, even if they don’t know it.
And then he starts getting vision of other people in trouble and he just has to do something to help them. And one of them is a desperate woman trying to cure her daughter’s cancer, about to make a deal with a crossroads demon. And the demon is old, and strong, but this is Sam fucking Winchester, and even at eleven years old, he’s Hell’s future king, and when the fight’s over there’s a dead demon, and Sam’s got a Hellhound, and a contract, and a woman’s very soul in his hands.
And first he tears up the contract (because he doesn’t know any better), but after the woman tearfully tells him that destroying the contract destroys the deal, he sits down right there and writes up a new contract as best he can. And he has to take her soul, because that’s how the contracts work, but he promises the woman that he’ll keep her soul safe until he’s allowed to give it back, promises her that she’ll have a full, happy life with her daughter.
Then he takes his new Hellhound and goes back to the hotel and he studies every legal textbook he can get his hands on. Contract law, especially, refining the contracts he writes for the people he saves, rewriting existing contracts held by crossroads demons that he kills.
He manages to keep his extracurricular activities secret from Dean and John for almost two years. (They’ve never shown an interest in his studies before, why would they start now?) And then they move to a new town for a hunt, and Sam wakes up in the middle of the night with a vision of a crossroad summons, and he doesn’t see Dean following him as he sneaks out of the hotel room.
He’s been summoned not by the usual method, but by a young girl’s sheer desperation. She’s crying as she sits on a swingset, a dark bruise on her cheek, more on her wrists, and Sam sits down on the swing next to her and whispers, “I can help you.”
“Can you kill him?” the girls asks.
“No,” Sam tells her, “but I can make sure he doesn’t hurt you, or anyone else, ever again.”
“What do I do?” the girl asks, and Sam holds out a contract.
“Give me your soul,” he says, “and in ten years, I’ll find you and give it back.”
It’s not even a question for the girl; she signs the contract with hands that shake from gripping the pen so hard. And Sam lets his Hellhound off her leash, watches her disappear in a puff of smoke as she goes after the girl’s father. When she returns a few minutes later, Sam scratches her behind the ears and tells the girl, “You’re safe, now. I promise.”
Then he runs back to the hotel, intent on getting back before Dean and John notice him missing. Only he’s too late, and they’re waiting for him on the other side of the door, and Sam is strong, but they’re his family and he can’t hurt them, can’t even fight them. (When John calls him a monster, he’s not even sure he wants to fight.)
He spends the next several hours tied to a chair while John uses every exorcism at his disposal. Sam begs, and pleads, and screams until his throat is raw and his voice is gone, but his father is resolute. Sam’s Hellhound fled as soon as she could, whimpering in pain from the exorcisms, and Sam hopes that wherever she is, she’s gone somewhere where she’ll be safe and loved. (He wishes he could do the same, but there’s no safe place for him anymore.)
But then John steps outside to take a break, takes Dean with him so Sam can’t corrupt him, and Sam’s alone in the hotel room. And then the sound of a window creaking open, and the girl from earlier slips out of the bathroom and creeps over to where he’s tied to the chair.
“Your dog came to get me,” she whispers, as she works at the knots. “She’s distracting those two so we can sneak out the window.”
“I don’t have anywhere to go,” Sam protests, and the girl gives him a small, sad smile.
“Neither do I,” she admits. “But anywhere’s gotta be better than here, right?”
When the ropes fall, Sam follows the girl out the bathroom window and across the parking lot to the woods on the far edge. They’re deep inside when the Hellhound joins them, jumping happily on Sam and licking him all over. Sam hugs her in relief.
“Thank you,” he tells the Hound, and then repeats it to the girl. “Thank you. I’m Sam Winchester, by the way.”