The Mirror's Reflection
Cordelia's wish to get rid of Buffy backfires. The town becomes a dark version of itself, and original timeline is severed and mutilated. The only way to restore the right reality is to find Anyanka - who isn't that available.
The beloved and known characters become vile reflections of themselves. Willow and Xander terrorize the Sunnydale as vampires, Angel is a vampire's captive and Buffy... lost her sense of purpose - as nothing can stop the bloodsucker plague.