The Set of a Sun and the Rise of a new One (MLP fanfic w/th ocs)

What happens when Celestia loses control over the sun? Twilight and her friends are sent to discover who has taken over the sun. But do they get more than they bargained for when they discover a whole other kingdom that is larger then Equestria? What is this new kingdom called? Will Celestia gain control over the sun again? Or will Equestria fall to the new Kingdom?