Dracula

Serial killer as a human, vampirism was a result of dark magic
19th century: Story was published in Bram Stoker's novel Dracula
2000: Went to Sunnydale to meet Buffy Summers, the Slayer;
Hypnotized Xander into becoming his servant, battled & was defeated by the Slayer
Both an enemy & ally to the Scooby Gang, maintained an unusual friendship with Xander