The book of fours - Nancy Holder

From a place of nightmares familiar both to Buffy and to Faith, a terrible evil invades Sunnydale, setting off a quartet of disasters: a forest fire, a tidal wave, a hurricane and an earthquake. Earth, air, fire and water -- mirrored in the Slayers' dreams by four different figures trudging across a pristine desert, each one carrying a box made of skin and bone. Faith thinks she has seen the box before, and research leads Giles to the Watchers' Diaries, where he learns that the last Slayer to encounter such a box was India Cohen: Buffy's immediate predecessor.
Buffy, who strangely has never considered the Slayer whose death activated her own calling, is eager to contact India's Watcher, Kit Bothwell. Kit, in turn, is intrigued by the prospect of working with two living Slayers at the same time. What's more, he has honed his sorcery skills over many years, and believes that he can enable Buffy to communicate with the dead India -- amongst others. Thus begins a journey through history that leads Buffy eventually to confront an evil that predates even humanity itself...