Be it that he bought you at an auction, stole you from your parents, saved you from the gallows, found you alone in the forest or you've been raised by him for this purpose; you find yourself belonging to him, his wishes for you to follow the craft, cult or family is what keeps you fed, warm and protected from all the evils in the world but those practices have a price that he takes from you every night...
Fantasy, horror or similar setting, mainly ageplay but O'dlike some master x slave/pet, magic, incest (maybe) and dub/non-con with it.
Alternative idea: The Homunculus.
An alchemy crafted human being, a flesh (or other) golem, an automaton or some other construct, you've been created to serve or maybe your purpose was corrupted so; maybe you were once human or have the soul of one, maybe you were summoned like a demon or spirit and bound to obey or at least forced to by magic, maybe you're just a humanoid species but this cruel world treats you like property, anyways you belong to someone or maybe some group and future seems dark to you, do you accept it, love your purpose or want to be free?
Last one, just a modern objectification, maybe by legal means (some new law that allows for treating someone like a thing and not a human being) or illegally by human traficking.