I think that's right though it could just be close. Its an intense one on one staged firht. maybe some sort if ritual. I think her friends are watching. I know its super serious scene. like they're fighting for their lives. Or maybe their freedom? I think whoever wins gains complete autonomy over the other. no free will.

Oh man, I don't know if it's the same story, but sounds like something I remember reading ages ago and haven't been able to find since. If it's the same, I think I read it on fanfiction.net. The fight was at the start of a long story, and Buffy won and claimed Spike and had some kind of anger issues later?

YES. she took in the power of the slayer demon maybe? I think i read it here forever ago. and she starts abusing him which is scary. she can't control herself and its sad because she owns him and people are freaking out and the slayer entity is taking over her brain and i think they need Dawn to stop her.

Clearly you triggered my memory

anyone know this story?

It sounds like it might be Blood Bound by Dreams of Spike? https://dark-solace.org/elysian/viewstory.php?sid=1175