Original sins - Christopher Golden

The future in which she finds herself is worse than Buffy could ever have imagined. The whole of Southern California has fallen under vampire rule, with Rupert Giles its Vampire King. Joyce Summers is dead. Spike has been slain and Faith, rogue Slayer, is gone. And Buffy must cope with the knowledge that she herself created this terrible future: her own insecurities and jealousies led her here. Now she must gather her inner strength and combat the demon that inhabits the corpse of her most trusted mentor.
Can she defeat a foe who not only knows her techniques, but who taught her, trained her, and helped her develop her skills? And even with the help of Willow and the Slayerettes, can she return to her real-time body? Only if she can identify and then rectify the one wrong moment that brought this future to pass...