Buffy The Vampire Slayer S03E09

The Wish

 Cordelia's pain over Xander's betrayal summons Anyanka (Emma Caulfield), a vengeance demon who grants wishes to scorned women. Cordelia wishes that Buffy had never come to Sunnydale, and constructs an alternate reality where the Master (Mark Metcalf) rose from the dead and the Harvest took over Sunnydale. Giles discovers this in the alternate reality and realizes he is able to reverse the events.