[s1e7] Something Blue Here
While grieving her breakup with Oz, Willow attempts a "Truth Spell" to ease her pain, but her grief-stricken intentions cause the spell to backfire. Instead of finding clarity, her every spoken word becomes a literal reality. This leads to the famous comedic subplot where Buffy and Spike suddenly believe they are madly in love and decide to get married.
goes blind because Willow mentions he "can't see anything." Xander becomes a literal "demon magnet." [S1E7] Something Blue
If you are looking for the specific text of the spell or the props used, here are the key details: While grieving her breakup with Oz, Willow attempts