|Although this article is based on canonical information, the actual name of this subject is conjectural.|
- "- Herein lies your truest strength.
- The energy of the Demon. Its spirit.
- Its heart.
-This is how you...
- Created the First Slayer? Yes."
- ―The Shadow Men and Buffy Summers[src]
The Shadow demon was an ancient demon used by the Shadow Men to create the First Slayer. They chained Sineya to the ground so that she could neither move nor escape, then unleashed the demon, which appeared to be in the form of dark black energy vapors. It forcibly merged itself with Sineya, granting her superhuman strength, speed, reflexes, agility, superior healing factor, a predator instinct, innately formidable combat techniques and quite possibly some kind of advanced prophetic ability. However, the process also caused the First Slayer to lose touch with her own humanity forever.
Thousands of years later, the Shadow Men tried to repeat history with Buffy Summers in order to empower her even more so she would be able to defeat the Turok-Han army of the First Evil. However, Buffy sought knowledge, not power, especially if that power meant that she would lose her humanity.