A Slayer, in the television series Buffy the Vampire Slayer and Angel (both created by Joss Whedon), is a young female bestowed with mystical powers that originate from the essence of a pure-demon, which gives her superhuman senses, strength, agility, resilience and speed in the fight against forces of darkness. She occasionally receives prophetic dreams in the few hours that she sleeps.
The opening narration in the Buffy series states "Into every generation a slayer is born: one girl in all the world, a chosen one. She alone will wield the strength and skill to fight the vampires, demons, and the forces of darkness; to stop the spread of their evil and the swell of their number. She is the Slayer."
While, in the series, they are commonly referred to as "Vampire Slayers", even by Watchers and vampires themselves, the Slayer may operate as a defender against any and all supernatural threats.
The reputation of the Slayer is well-known and revered, even throughout other dimensions. The notion of The Slayer has been compared to the equivalent of a Demonic "Boogey-Man," incredibly feared and considered by most to be essentially unconquerable.
In every generation there is a Chosen One. She alone will stand against the vampires, the demons, and the forces of darkness. She is the Slayer.
Thousands of years ago in ancient Africa, a group of tribal elders known as the Shadow Men used powerful magic to infuse a captive girl with the heart, soul and spirit of a demon known as the Shadow Demon. This process granted the girl great strength, speed, agility, reflexes, healing, endurance and psychic abilities in the fight against evil she became the First Slayer, called to fight the vampires and other demons that populate the Earth. The Shadow Men's descendants went on to form the Watchers' Council, an organization dedicated to finding, training, and supporting Slayers.