Buffy the Vampire Slayer Season 6 Episode 13 : Dead Things

The Troika's latest escapade to make Warren's ex-girlfriend Katrina their willing slave goes horribly wrong when she is accidentally killed. The trio use magical and demonic forces to make Buffy think that she has killed Katrina - and it works. With finding out the truth about her resurrection and believing she killed an innocent girl, Buffy's world begins to fall apart.