Buffy The Vampire Slayer S06E17

Normal Again

Warren, Jonathan, and Andrew unleash a demon whose powers make Buffy believe that her friends are figments of her imagination. Buffy attempts to attack her friends, locks them in her basement and unleashes the same demon on them. Tara eventually finds the trio and Buffy recovers from the demon's poison by drinking an antidote. By the end of the episode however, it is unclear which world is reality.