Dig a bit into the history of this place," as librarian Rupert Giles explains to Buffy, "You'll find there've been a steady stream of fairly odd occurrences." The area is a center of mystical energy. And all the signs point to a crucial mystical upheaval. Some sixty years ago a very old, very powerful vampire came to this southern California town, and not just to feed. The Spanish who first settled the area called it Boca Del Infierno -- roughly translated, "Hellmouth" -- a sort of portal from this reality to the next. This Master Vampire hoped to open the portal and bring the demons back. And destroy the world of Man.
"But," as Buffy's new friend Willow Rosenberg adds, based on her own computer research into old city and county records, "he blew it. There was an earthquake that swallowed half the town. And him too -- or at least there were no more vampire-type killings afterward."
"Opening dimensional portals," Giles adds, "is tricky business. Odds are he got himself stuck. Like a cork in a bottle."
However, once in a century comes The Harvest: a night when a Master Vampire can draw power from one of his minions while it feeds. Enough power to break free, and to open the portal. The minion is called the Vessel, and he bears a three-pointed star symbol.