The Master

Very little is known about The Master, but what we do know is that "some sixty years ago, a very old, very powerful vampire came to these shores, and not just to feed". For the whole first season, The Master was Buffy's main enemy in the fight of good versus evil. He is roughly 600 years old, and was originally drawn to Sunnydale because it it situated directly over the Hellmouth, which Giles described as "a sort of portal between this world and the next". The Master hoped to open the Hellmouth, allowing demons to walk the world once more, but he was foiled by the 1937 Sunnydale earthquake, which trapped him in an old church, which was ironically to become his lair.

He remained in limbo there until 1997, when he managed to escape at the end of the first series. A prophecy in the Pergamum Codex foretold a battle between the Master and the Slayer, which would cause the Slayer's death. After some deliberation, Buffy descended into the Master's lair, and was bitten by him. The Master had now become free, and dumped the Slayer's body in a puddle, where she drowned, although Xander performed CPR on her and revived her. As soon as she was recovered, Buffy raced after her old foe and caught up with him:

The Master: You were destined to die - it was written!
Buffy: Guess I flunked the written.