The Meme of Bilitis

Witch/familiar setup where the familiar essentially swears herself into magical servitude in exchange for protection. The familiar serves as a kind of magical "amplifier" that makes the witch more powerful and also gets access to some magic that she can use herself from the runoff. Witches basically have free rein over their familiars until death, but it's until one of them dies - you can't inherit a familiar - and this is someone you just made magical and who will undoubtedly find out all your secrets and weaknesses, so abusing a familiar past the point they can endure is not advisable. (On the other hand, life is rough and familiars are willing to endure a lot, better the devil you know, etc.) The bond is sealed with sex and witches treat their familiars in a bunch of different ways - like treasured pets, like sex slaves, like roommates, like friends, like lovers, like a necessary inconvenience.

I feel like there's a lot of flexibility here to exploit, and I love it when slaves aren't totally powerless but are SUPREMELY disadvantaged nonetheless. You can have slave kink AND loyalty kink all in one! Plus, I like magic and all its sexy potential.