Ancient Origins: A Power Divided

A power is rising, a power so frightening that it threatens all of existence, even Arceus fears its return. With the help of the 18 Elementals, Arceus defeated this creature but barely. Ash and Serena's lives change drastically one day when they find out just how important they are in helping Arceus prevent the return of the ultimate evil, a creature so dark that, like Giratina, was banned to its own dimension. What role do the two of them play, what connection do they have to all of this and how will they prevent the destruction of reality itself?
This is an Amourshipping story and my next crack at a big story.