Primeval slayer was the first slayer in the fictional Buffy the Vampire Slayer television series. She was created by a group of men thousands of years ago by mystically implanting the heart of a demon into a young girl.
Buffy Summers has encountered the primeval slayer on several occasions. The first after using a powerful spell to defeat the Initiative-created demon/human/machine hybrid Adam. The spell had the side effect of sending her and her friends into a mystical dream where they were confronted with the primeval slayer.
Another occasion was when Buffy went on a vision quest to find her destiny. The first slayer helped her to discover that her gift was death.