BTVS: TALES OF THE SLAYERS
For untold eons, demons walked the Earth, making it their home and Hell. In time, they lost their purchase on this reality, and the way was made for mortal animals. For Man. What remains of the Old Ones are vestiges: certain magicks, certain creatures… The last demon to leave this reality fed off a human, mixed their blood. He was a human form possessed- infected- by the demon's soul. He bit another and another… and so they walk the Earth, feeding. Killing some, mixing their blood with others to make more of their kind. Waiting for the animals to die out and the Old Ones to return.
The vampires were a plague. The elders of several villages met, calling for action. They invoked the strongest and most dangerous magicks they could summon, to create a power, a power that could fight the vampires. A power that lived in the body of a girl. The first Slayer was born. She fought, and died, and was succeeded by another, and another. Throughout time, always one. Always a woman. A warrior. They were trained, sought out and guided by Watchers, descendants of the shamans who created the first Slayer.
The Slayer hunted and killed vampires, but her own people mistook her for a demon, due to her superhuman powers, and expelled her. The Shadow-Men failed to find her and so she never had someone to guide her. The Shadow-Men's descendants became the Watchers and eventually the Watchers' Council. Eventually, the Slayer died and her power passed on to another girl. The Slayer lineage had begun.
The Primitive lived on in all the future Slayers, and manifested in the dreams of 21st century Slayer Buffy Summers. When the town of Sunnydale was devoured by the Hellmouth and Buffy set herself on a "damage collision course," the Primitive intervened, showing Buffy that her duty was not about slaying, but about saving instead.