THE DARK SIDE. BtVS Intertext/Allusion.
In the episode “Choices”, Buffy refers to Faith’s defection to the Mayor’s camp as turning to the “dark side of the Force”, as she explains to Willow why she can’t attend an out of state school after high school graduation despite Joyce Summers’ excitement about Buffy’s acceptance at a prestigious school.
Willow: Sounds like your mother is in a state of denial.
Buffy: More like a continent, She just has to realize that I can’t go away.
Willow: Well, maybe not now, but soon, maybe. Or maybe I too hail from Denial Land.
Buffy: Faith’s turn to the dark side of the Force pretty much put the proverbial kibosh on any away plans for me. UC Sunnydale – at least I got in. You! I mean I can’t believe you got into Oxford!
In the Star Wars universe, Jedi knights (a noble order of protectors who arguably fulfill a function much like Slayers in the Buffyverse) derive their powers from their belief and observance of the Force, an energy field created by all living things that binds the galaxy together. Jedi use the Force for knowledge and defense against the Dark Side, the powerful, forbidden negative energy that can seduce less disciplined Jedi.
In Season Seven, Andrew, one of the Troika who aspired to be Buffy's "nemeseses" in Season Six, admits in "Bring on the Night" that he has tasted the Dark Side:
ANDREW: OK, I know what you're thinking. Andrew, bad guy. You think I'm a super-villain like Dr. Doom or Apocalypse or-or The Riddler. But I admit I went over to the dark side, but just to pick up a few things, a-and now I'm back. I've learned. I'm good again.
BUFFY: And when were you good before?
ANDREW: OK, technically, never. Touché. But I'm like Vader in the last 5 minutes of Jedi with redemptive powers minus a redemptive struggle of epic redemption which chronicles... These ropes itch.