Kind of sterek black butler au?
After the brutal murder of the Sheriff, Stiles is left orphaned. The very system his father served turns a blind eye, burying evidence and protecting the obvious culprits. Angry, and desperate for answers, Stiles turns to the occult like any other 14 year old teenager. He tries to summon a demon, offering his soul on a silver – eh, he didn't have silver, it was just a plain porcelain platter.
What he gets isn't exactly a demon.
Instead, a massive, black wolf appears, one that speaks in a low, human voice. It tells Stiles it isn't fully real, trapped in an existential plane where time doesn’t pass. It remembers nothing of who it was or how it got there, only that it cannot leave unless it trades places with another soul.
Stiles, reckless and aching, and also just a fucking kid, so he offers his soul. But he has conditions, though: the creature must help uncover the truth, the case his father was investigating, and bring the man justice while protecting Stiles from being thrown into foster care. The wolf agrees.
In the blink of an eye, the beast shifts into a man. He's dark-haired, tall, with unsettling seafoam green eyes. When Stiles demands an explanation, the man shrugs. It's the first time he’s taken human form since slipping from limbo, and he has no idea why this face – this body – was what emerged.
For now, Stiles "owns" him. The man can shift at will, bound by their contract.
“What’s your name?” Stiles asks.
“I don’t have one,” the wolf-man replies.
“Balto,” Stiles declares with a nod. “Because you’re my dog now.”
“I’ll stick to Derek,” the man growls through clenched teeth. “My lord,” he adds, mocking.
“That’s such a boring name,” Stiles frowns. “Sebastian, then.”