Lothos

Lothos was a vampire king based in Los Angeles, allegedly around eight centuries old and claiming to have been responsible for the deaths of dozens of Slayers. Having tracked the latest Slayer- the newly-called Buffy Summers- he attempted to capture her Watcher Merrick to learn who she was, but Merrick committed suicide rather than risk giving away vital information. Unfortunately, one of Lothos's new subjects was a classmate of Buffy's in life, and was thus able to direct Lothos to the gym where Buffy's school dance was taking place. In a final confrontation outside the gym, Buffy torched Lothos by tricking him into touching a cross- subsequently using her hairspray as a makeshift flamethrower- and then staking him, proclaiming his defeat as being both for Merrick and for the girl she'd been before becoming the Slayer.

Canonical
"The Origin"

Non-canonical
Buffy the Vampire Slayer