Portal through time - Alice Henderson

The Master, the unimaginably ancient and powerful being of evil who dwelt in the Hellmouth underneath Sunnydale High, was Buffy's first great challenge. Once he almost defeated her, leaving her for dead before she finally overcame him. A vampire lord of his dread age and stature is bound to leave acolytes behind him, however. The Master may be gone, but he is by no means forgotten.
One of his devotees has set out to alter the past, killing off Slayers in order to change the future. With the Slayer line extinguished, there will be no Buffy. And without Buffy, no one will be able to prevent him as he resurrects The Master, bringing back his lord and king in the fullness of his power. Already he has conjured up a portal through time to send his minions into the past _" poised to murder the most influential Slayers in history. The Scoobies are forced to follow them through the portal in a wild race against time and evil. But the problem is, you can't change the past without altering the present...