Immortal by Christopher Golden
During a routine slaying, Buffy Summers runs into a small group of vampires who seem to be out for more than the usual bite-and-suck. As the vamps scatter, one of them stays behind, willing to sacrifice itself to help the others escape. Though surprised -- vampires aren't exactly known for their generosity -- the Slayer doesn't waste time pondering the demon's motivation and kills it.
Or so she thinks.
But the next night a new vampire confronts the Chosen One and her friends. One who knows Buffy's name and can anticipate her basic attack. One who doesn't have the proper respect for Mr. Pointy. One whose name is Veronique.
"She cannot die. Strike her down, but like the night she will always come again. And she will bring forth the end of Man...."
Every time Buffy kills her, Veronique rises again in a new body.
"Great. Instant replay." An invincible demon is the last thing the Slayer needs right now. Joyce Summers is about to undergo a serious operation, calling up all of her daughter's fears about her own mortality. Angel wants to comfort Buffy, but her mother's crisis underscores the difference between them: he will live forever, while she will grow old...or die young.
Torn between her duties as a daughter and as the Chosen One, Buffy will need the support of her friends to help her solve a rash of grave robberies, head off an influx ofnew vampires, and take Veronique down, once and for all.
For Veronique is on a mission -- to bring about the unification of her masters. the Triumverate, into one all-powerful demon who will drink the blood of the last man on earth....