The Dark Slayer was a duplicate of the real Buffy Summers created by the infamous alchemist Doctor Flitter.
History
The Dark Slayer was created by Doctor Flitter using the bone of a dead Slayer taken from the Kien-Ju vampire sect, a polaroid of Buffy Summers, and the blood of the vampire Selke. Though his first two attempts were failures, the third was a success.
The Dark Slayer caught the real Buffy unprepared during one of her nightly patrols, and quickly overpowered and seemingly killed her, dumping Buffy's body in the sewers. She proceeded to assume Buffy's identity and attempt to kill her friends before discovering that Buffy had survived their encounter.
The Dark Slayer proceeded to track Buffy down in the sewers and did battle with her again. Though she briefly gained the upper hand, the appearance of one of the failed Dark Slayer clones gave Buffy the advantage she needed to knock them into the sewer water, apparently killing them both.
Physiology and powers
The Dark Slayer possessed the typical powers of a Slayer, including superhuman strength, speed, agility, reflexes, and durability. During their first encounter, she overpowered Buffy with ease.
As a side effect of the vampire blood used to create her, the Dark Slayer had no reflection and also possessed an accelerated aging process that rendered her unable to heal any wounds inflicted on her.