Somehow or another, it could be that the First Evil has slipped 'a nasty little something' into magic, but Tara get's addicted and uses some sort of spell to enslave Buffy!
This is admittedly messed up! Very messed up! (you kind-of get the picture) and the concept is both Dark and rather gross! (considering Tara's orientation and what 'physical intimacy' under the influence of magic is in that universe..., as well as the corruptions slave-holding societies tend to wind up with)
It can be construed that while alive she lost her soul to Gehanna, and her friends are trying to bring the young woman to her senses- that could be a touch.
Must include: Tara going bad, Buffy winding up abused by Tara, hidden enslavement, mind-control.
Can include: The First Evil tempting Tara to enslave Buffy, Tara mad at Buffy for some reason, (my reading on the First's personality is that that spirit is profoundly opprotunistic, and will also strike if somebody is vulnerable to it) Sadistic Tara, Traumatised Buffy.
Should Never Include: pornography (I won't tolerate that).
Improbable ideas: (hey, the whole premise is improbable, so...) Tara is a smart woman, so I seriously doubt that she would ever impliment a gang to abuse Buffy (hey, considering how sweet she is, this is exceedingly unlikely, but it Does lead credance to the concept that Tara is possessed- maybe she is in this tale!), but the basic concept is that Tara enslaves Buffy in EVERY way! I doubt that she'd use whips or BDSM gear, either... but you never know.
Suggestions: you could have Tara get so mad at Buffy that she makes a deal with the First, and those always go bad! So, The First possibly possesses Tara when she invites 'him' into her body for extra strength; only to find (for her, that is) that this is a really raw deal! That COULD explain the mixed up nature of this story! Hey, maybe Buffy and Tara got into a fight over Tara's orientation, and Tara decides to 'make Buffy see the light' as it were. (just a thought)
Challenger hopes these will be included: Tara repenting before the end of the story, and getting along with Buffy. Buffy and Tara becoming friends again. Upset Dawn accepting Tara's apology, as well as Buffy doing so, maybe Tara taking them out to dinner as a peace-offering.
Possibilities: laughing Spike (when he learns about the discord in the Scoobies.), the First happy about this.
Required: enjoy writing the story, challenge will remain open for as long as humanly possible!