BIG BAD. Buffyverse Term/Concept/Idea/Intratext.
In "Wild at Heart," Spike's self-designation (just before he is captured by The Initiative).
Spike : Watch your mouth, little girl. You should know better than to tempt the fates that way. 'Cause the big bad is back . . .
Since that moment in Season Four, the term has been regularly used to describe the major foe Buffy and the Scooby Gang must face each season. Each of BtVS' seven seasons had a Big Bad:
Season One: The Master
Season Two: Angelus
Season Three: Mayor Wilkins
Season Four: Adam
Season Five: Glory
Season Six: Dark Willow
Season Seven: The First Evil