The book of fours

Buffy tells Giles of her visions of disasters involving earth, air, fire, and water and of a primitive figure holding a box made of human skin and bone. Giles finds the last Slayer to see such a box was Buffy's predecessor, India Cohen. Teaming up with India's Watcher, a sorcerer named Kit Bothwell, Buffy sets off on a history-spanning journey that forces her to confront her own legacy as the sole inheritor of a Slayer continuum.