Sineya
Disclaimer: I don't own Buffy the Vampire Slayer. Buffy the Vampire Slayer is an American television series that aired from March 10, 1997, until May 20, 2003. The series was created in 1997 by writer-director Joss Whedon under his production tag, Mutant Enemy Productions with later co-executive producers being Jane Espenson, David Fury, David Greenwalt, Doug Petrie, Marti Noxon, and David Solomon.

The First Slayer:
In ancient Africa, a group of tribal elders known as the Shadow Men used powerful magic to infuse a captive girl named Sineya with the heart, soul and spirit of a demon.[1] This process granted the girl great strength, agility, and endurance: 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.[2] This is Her story.