According to the Eye of Beljoxa, Buffy's resurrection triggered the rise of the First Evil, a revelation which would have presented an alternate means of winning the war, if anyone had thought of it. What if someone did?

With the right witches to make it happen, time travel should be simple: Go back. Change only what needs to change. Act normal. But Buffy doesn't travel light. The weight of her loves, losses, hopes, and regrets goes wherever she does. And so does her vampire.