Blood and fog - Nancy Holder

The strongest magic ever distilled, and the deadliest butcher England has ever known... Buffy is on the trail of a killer demon, and reluctantly needs Spike's help. But Spike, as usual, has an agenda of his own. The demon carries vial of pure magical power, and Spike can think of several people -- or demons for that matter -- who would pay the earth to have it in their possession. Doc, for instance. Or Rack. Or the nameless evil known only as The First. Spike has met The First before. In the good old days in Victorian London when he, Drusilla, Angelus and Darla ran through the night in pursuit of unholy fun, another terror also stalked the streets. And when the so-called 'Jack the Ripper' struck too close for comfort, the vampires found themselves on the same side as the Slayer of that time. Working to bring down Jack, and running afoul of The First, Spike and the Slayer formed an uneasy alliance: a bond which has tracked him down the years to twenty-first century Sunnydale, now blanketed in an eerie, mysterious fog...