"Bad Girls" is episode fourteen of season three of the television series Buffy the Vampire Slayer.
The next day in the school library, Buffy's supercilious new Watcher, Wesley Wyndam-Pryce, identifies the swords as belonging to a cult of swordsman vampires, who were once led by a demon named Balthazar.
Faith impulsively jumps into the vampire nest and is joined in the fight by Buffy, who finds the amulet.
Buffy and Faith find Balthazar (an obese demon who lives in a vat of water) surrounded by his vampires, so they break into a sports equipment store to steal weapons.
Buffy runs into Angel who tells her about Balthazar capturing Giles, while Faith returns to Allan's body.
[1] After Faith accidentally kills Allan, Buffy finds her in her hotel room obsessively scrubbing his blood from her shirt.
[2] Vox, rating it at #26 in the list of all 144 episodes, writes, "With Faith embracing the role of Buffy’s Evil Twin Slayer counterpart, this episode lets Buffy embrace the wild streak inherent in being a teen with superpowers, as she and Faith go slaying and dancing whenever the hell they feel like it.
"[3] A review for the BBC said the scene where Allan is killed "signals a fundamental change to a more mature series".