In every world there is usually one or two great cultures that existed before the present, peoples who spanned across the world or had technology that no one could explain. Civilisations that, for reasons never fully realised, faded away into the avenues of the past. There are always rumours, stories and myths about them, and their resurgence or discovery always adds a nice element of background to any world.
When a great civilisation falls into ruin, their markings are always left of the world somewhere, in the form of ruins, of lost trinkets, and in the histories written by those that came after them. There are usually descendants of this empire somewhere, people who remember the "true history" of the world, with aspirations of rebuilding the empire or getting revenge on those responsible for its downfall.
Lost in Time
A culture can be lost by being isolated, contained by their environment, and when they fade away no one even notices them. They are never recored into the histories of the other civilisations, lost in the annals of time until some explorer comes across their ruins… or a still thriving, living place. Either discovery can change the shape of a world for very different reasons.