- "We cannot give you knowledge. Only power."
- ―Shadow Man[src]
In prehistoric times, the Shadow Men chose a young girl, Sineya, to fight against the forces of evil. They chained her to the floor of a cave so that she could not escape, and infused her with the heart, soul, and spirit of a powerful demon known as the Shadow demon. At the cost of most of her humanity, the First Slayer was created. She had superhuman strength, speed, reflexes, agility, a predator instinct, superior healing factor and a psychological link which she could use to project herself into the dreams and visions of her successors.
Guided by her instincts, and armed with her superhuman powers, Sineya hunted and killed vampires and demons. But the very people of her village, whom she protected, mistook her for a demon and expelled her. The Shadow Men failed to find her, and hence she never had a Watcher.
However, the descendants of the Shadow Men and their subsequent apprentices would later form the Watchers Council, which is responsible not only for identifying and training the Slayer in the fight against evil, but also for identifying the Potential Slayers and giving them training as well in preparation of the possibilities of any of them being called.
When Buffy attempted to communicate with the Shadow Men through a mystical set of shadow casters, they tried to give her more power by repeating the process through which they created the First Slayer eons ago: infusing Buffy with the Shadow demon. However, Buffy rejected it as she sought knowledge instead of power, and she knew that the cost of her acquiring more power would be her humanity, just as it was for Sineya.
The Shadow Men were forced to yield to her determination, but before she was sent back, they gave her a vision of the Turok-Han vampire army, which led her to think that she may have made a mistake by rejecting the power they offered her.
The Shadow Men were directly responsible not just for the creation of the First Slayer, but also for establishing the line of Slayers as well as the mystical forces that govern a Slayer's activation.
Besides this, their magic also created a great number of Potential Slayers, and each of them would experience vivid nightmarish dreams of the heroics of past Slayers, but only one in each generation will be chosen.