I want to play a character who dedicates themselves to a high powered being, who in return for me doing their bidding grants me magical abilities.
Am I describing WARLOCK or CLERIC?
Cleric, as they're dedicated
Try to do any of these blatant warlock things as a cleric and see how well it goes
Researcher who stumbled across an ancient tome in a library. Upon opening it, their mind was exposed to that of something beyond, from which they took a sliver of power. But they now know you're there, and their great eyes that should not be come ever closer...
Gambler who gambled far too much until they made a bet they couldn't win. Someone came to help them, but now they're in debt to someone they'd rather not be. In order to ensure their debt gets paid, well, this wealthy patron has sent an imp or two to make sure you're in a position to pay up.
You had a one-night stand with a beautiful woman. Next thing you know, you've forgotten the last 2 months of your life and you have strange new powers, tied to the ever-flowing power of life and nature. You're pretty sure she's got a vested interest in you, and, hey, how were you to know she was actually a Fey and so help you if you go to the Feywild?