BTVS: TALES OF THE SLAYERS
Yuki Makimura was a Slayer who was turned by the powerful vampire The Master. As a vampire, Yuki gave her complete loyalty to the Master, prompting The Watcher's Council, who were so disgraced by this turn of events, to erase all records of her from their records and journals. She served the Master from her station in Japan until learning of his death in 1997 at the hands of Buffy Summers.
Yuki later ventured to Sunnydale where the Master's essence remained, and kidnapped one of Buffy's friends to use as a vessel for the Master to inhabit. Buffy and her friends interrupted the ritual and used a powerful weapon, a vapour blade, to destroy the Master's essence. Blinded by her rage, Yuki became careless and gave Buffy the winning edge she needed to dust Yuki.